Geochemical Characteristics and Environmental Implications of Trace Elements of the Paleocene in the West Lishui Sag, East China Sea Basin

Abstract

Analysis of the sedimentary environment during the clastic formation process is of great significance for reservoir evaluation and desert prediction. This paper focused on the Paleocene in the West Lishui Sag, East China Sea Basin. XRF fluorescence diffraction and carbon and oxygen isotope tests were carried out on core samples from four wells. Based on the geochemical characteristics of the samples and the changes in the elemental ratios, combined with the lithologic characteristics and sedimentary structure of the samples, the paleoclimate, paleosalinity, paleobathymetric, paleoredox, paleotemperature, and other paleoenvironmental characteristics were analyzed. The results show that the characteristics of major and trace elements were similar in the lower Mingyuefeng Formation (E₁m²), Upper Lingfeng Formation (E₁l¹), Lower Lingfing Formation (E₁l²), and Yueguifeng Formation (E₁y). The Paleocene in the West Lishui Sag was mainly in the reducing environment of brackish-salt water with weak water stratification. The water depth showed a trend of becoming deeper, then shallower, and then deeper. The paleoclimate in the West Lishui Sag was warm on the whole. However, the content of Sr became smaller after later deposition, so the calculated paleowater temperature was higher. In addition, oxygen isotopes were affected by diagenesis, resulting in a negative oxygen isotope value. The paleoproductivity was low, and the hydrocarbon generation potential was poor. The content of nutrient elements mainly came from terrigenous input rather than biological origin, and terrigenous intrusion characteristics gradually increased from E₁y to E₁m². The study also shows that paleoproductivity was affected by the paleoclimate and paleowater depth. Warm and humid climate and deep water body were conducive to the accumulation of paleoproductivity.

1. Introduction

Trace elements in sedimentary rocks are closely related to their forming environment [1,2,3]. During the sedimentation process, there is a complex geochemical balance between the sediment and water medium such as element exchange between the sediment and water medium, and the adsorption of some elements by sediment, etc. [4]. The exchange and adsorption of these elements are affected by the physical and chemical conditions of the sediment media. Water media in different sedimentary environments have different physical and chemical conditions, and trace elements and their content in sedimentary rocks are highly sensitive to the changes in water media in sedimentary environments, which can provide reliable information on the changes in the paleoenvironment and paleoclimate [5,6,7]. Therefore, by studying the content and distribution characteristics of trace elements, especially the ratios of some related elements, the sedimentary environment at that time can be inferred and the geological conditions of the sedimentary period can be inverted [8,9,10,11].

In recent years, industrial oil and gas have been obtained in the West Lishui Sag, East China Sea Basin. A lot of research has been conducted on the reservoir characteristics in this area, but mostly from the perspective of lithology and color qualitatively, with relatively little attention paid to the sedimentary environment, while quantitative analysis is more helpful to predict high-quality reservoirs and deserts. Therefore, this paper focused on the E₁m², E₁l¹, E₁ml², and E₁y of the Paleocene in the West Lishui Sag, and systematically carried out elemental geochemistry research including the major trace elements and the carbon and oxygen isotopes, and analyzed their relationship with the paleoenvironment. The sedimentary paleoenvironment has been quantitatively evaluated by using the geochemical characteristics of elements and the ratio between elements, combined with the lithologic characteristics and sedimentary structure, and its changing trend was also analyzed, in order to provide new ideas for desert prediction and petroleum exploration in the next step.

2. Geological Settings

2.1. Tectonic Evolution Characteristics

Lishui Sag is located in the southwest of the East China Sea shelf basin (Figure 1a,b), which generally extends from NE to SW. It is adjacent to the Qiantang Sag in the north, separated from the Fuzhou and Minjiang Sag in the eastern belt of the Taipei depression by the Yandang Low Uplift in the east, and connected with the Zhemin Uplift area in the west and south (Figure 1c). It is a typical Cenozoic fault depression developed on the basis of a Mesozoic residual basin, with an area of about 14,600 km². The NNE fault system of the sag can be divided into five tectonic units: the West Depression, the East Depression, Lingfeng Low Uplift, the South Depression, and the Yandang Uplift (Figure 1d).

Lishui Sag has experienced four tectonic stages including a rifting stage from the Late Cretaceous to Paleocene, a depression stage from the Eocene, an uplift stage from the Late Eocene to Early Miocene, and the regional subsidence stage from the Neogene to Quaternary (Figure 2). The rifting from the late Cretaceous to the Paleocene was characterized by episodic and progressive development, which can be further divided into four stages: initial rifting, intense rifting, stable development, and attenuation. The Yueguifeng Formation (E₁y), Lingfeng Formation (E₁l), Lower Mingyuefeng Formation (E₁m²), and Upper Mingyuefeng Formation (E₁m¹) of the Paleocene were formed in four structural stages [12].

2.2. Petrological and Mineralogical Characteristics

This study focused on the Lower Mingyuefeng Formation (E₁m²), Lingfeng Formation (E₁l), and Yueguifeng Formation (E₁y) of the Paleocene in the West Lishui Sag. Core photos indicate that E₁m² mostly consisted of gray massive sandstone and black mudstone, with thin interbeddings of gray fine sandstone and mudstone (Figure 3a), and thin coal seams were locally developed. The E₁l¹ was mainly composed of gray and gray-black thick mudstone (Figure 3b), interspersed with silty mudstone and fine siltstone, mixed with a small amount of thin sandstone. E₁l² was mainly gray-white fine sandstone, with a small amount of gray mudstone. Sandstone grains were slightly coarse. The Baoma sequence and slump structure could be seen (Figure 3c), which was considered to be caused by sandy debris flow. The E₁y was mainly composed of gray and gray-black mudstone, interbedded with thin layers of fine sandstone, and locally developed gray mudstones interbedded with siltstone or muddy siltstone, and wormholes and mesocosms could be observed in the layers (Figure 3d). The color of Paleocene sandstone in the West Lishui Sag is mainly gray and gray-brown, and the color of Paleocene mudstone is mainly black and gray-black, which indicates that the Paleocene in the West Lishui Sag was mainly in a reducing sedimentary environment. This is consistent with the transition from a semi-deep lacustrine sedimentary environment to a marine environment, as previously described.

According to the statistical analysis of the petrographic composition based on the cores from four wells and 137 thin sections in the West Lishui Sag, two rock types dominated each Paleocene series: lithic sandstone and feldspathic lithic sandstone (Figure 4). Among them, lithic sandstone was the main type, accounting for more than 95.0% of the total samples. Its petrographic composition was mainly lithic, with an average content of 59.8%, followed by quartz, with an average content of 19.0%, and feldspar at 13.4%. It is obvious in Figure 4 that the relative content of rock fragments in the sandstones of E₁m² and E₁l was mainly greater than 75%, while the relative content of rock fragments in the sandstones of the E₁y spans a larger range with the relative content greater than 50%. The particle size of debris was mainly distributed between 0.01 mm and 0.55 mm, mainly medium to fine particles, in a sub-prismatic sub-circular shape, medium sorting, and poor roundness. The interstitial materials were mainly composed of carbonate cement, clay matrix, authigenic quartz, etc. The average content of carbonate cement was 6%, mainly calcite, iron calcite (Figure 3e), and dawsonite (Figure 3f). The average content of the clay matrix was 2.1%, mainly illite (Figure 3g), followed by kaolinite (Figure 3h). Authigenic quartz is generally formed from the secondary enlargement of quartz, with an average content of 1.2%. Previous studies have shown that the main source of the Paleocene in the West Lishui Sag is from the Minzhe Uplift on the west side, which is characterized by rapid near-source accumulation and low structural and compositional maturity of the sandstones [13,14,15].

2.3. Sedimentary Characteristics

Based on core observation, single-well sedimentary facies and seismic facies interpretation, our predecessors have defined the types of Paleogene sedimentary system and sedimentary environment characteristics in the Lishui Sag [14,16,17]. On the whole, the Lishui Sag experienced the evolution process of transition from a continental faulted lake basin to a marine faulted-depression basin during the Paleocene (Figure 3 and Figure 5). The terrigenous supply was sufficient, the sandstone was well-developed, and the deposition thickness was large.

The E₁y was in the early stages of fault subsidence. It was generally a shore shallow lake–offshore deep lake sedimentary background, with semi-deep lake mud high-quality source rocks developed, and small deltas and fan deltas were locally formed. During the deposition period of the E₁l², the rifting in the Lishui Sag was further strengthened. At the same time, affected by the rise in the global sea level, seawater intruded rapidly from the south and east of the Lishui Sag, gradually converting it from a faulted lake basin to a marine basin. In this period, there were relatively few terrestrial inputs. During the deposition period of the E₁l¹, a second large-scale transgression occurred in the Lishui Sag, resulting in an expansion of the sea area, and thick littoral-shallow sea deposits were developed in large areas. Early deposition of the E₁m² coincided with the decline in the global sea level, resulting in incised valleys formed by rivers in the western gentle slope zone. At this time, the sediment supply was sufficient, so the delta front extended to the offshore slope and reached the offshore basin, and different types of delta deposits were developed in the gentle slope zone of the Lishui West sub sag. Afterward, the sea level gradually rose, and the delta of the western slope retreated to land, gradually surpassing the land of the previous stage. During the deposition period of the E₁m¹, the western depression zone expanded from west to east, and the sea area expanded, presenting an open littoral shallow sea environment, with the Lingfeng Uplift submerged underwater. At the end of the deposition of the E₁m¹, the transgression ended and large-scale seawater subsided [13,14,15,16,17,18].

3. Materials and Methods

3.1. Samples

A total of 30 mudstone samples were tested in this study, of which two were provided by the State Key Laboratory of Marine Geology for comparative analysis of the experimental results, and the remaining 28 samples were all taken from four drilling cores in the West Lishui Sag for paleoenvironment and paleoclimate restoration.

The samples cover the horizons from E₁m² to E₁y in the West Lishui Sag, East China Sea Basin. The well location, depth, and lithology of the samples are shown in

Table 1. The elemental contents in the West Lishui Sag.

Number	Well	Depth (m)	Formation	Lithology	Zr (ppm)	Sr (ppm)	Cu (ppm)	Ni (ppm)	Fe (%)	Mn (%)	Ti (%)	Ca (%)	K (%)	Ba (ppm)	P (%)	Mg (%)	Na (%)	Si (%)	Al (%)	Rb (ppm)	Zn (ppm)
1	A	2233	E1m2	mudstone	177.6	278.4	4.9	115.3	3.804	0.023	0.634	2.659	2.838	426	0.049	1.582	1.083	62.203	16.309	231.8	102.3
2	A	2234	E1m2	mudstone	255	193.8	4.7	130.3	2.188	0.014	0.751	0.189	3.258	420.6	0.049	0.698	1.181	66.127	18.812	220.4	99
3	A	2236	E1m2	mudstone	220.7	243	6.7	122	3.056	0.015	0.758	0.295	3.087	383.1	0.057	0.731	1.121	63.537	18.777	240.9	106.2
4	A	2257	E1m2	mudstone	223.9	145	7.6	652.1	3.357	0.02	0.745	0.159	4.367	509.7	0.054	0.87	1.361	61.587	19.284	282.8	99
5	A	2258	E1m2	mudstone	271.7	127.8	11.1	725	2.789	0.021	0.746	0.185	4.244	555.1	0.04	0.741	1.45	65.713	17.305	295	106.4
6	A	2285	E1m2	mudstone	237.5	205	8.4	22.2	3.356	0.018	0.578	0.755	3.982	583.1	0.058	1.053	1.513	69.401	14.283	260.1	79.6
7	A	2286	E1m2	mudstone	228.7	298.1	8.7	53.6	3.778	0.019	0.671	0.739	4.145	551.6	0.076	1.204	1.419	64.827	16.525	250.7	94.6
8	A	2287	E1m2	mudstone	240.2	234.7	8.3	28.7	3.365	0.021	0.541	0.942	3.877	563.8	0.063	1.258	1.409	69.389	13.862	244.5	83.2
9	A	2288	E1m2	mudstone	208.2	537.7	8.1	20.5	3.927	0.017	0.72	0.687	4.204	509.2	0.122	1.174	1.36	63.292	17.293	264	109.1
10	A	2293	E1m2	mudstone	337	163.7	8.4	234.3	4.902	0.026	1.109	0.144	4.816	603.1	0.074	1.066	1.233	54.87	18.544	352.4	82.3
11	A	2294	E1m2	mudstone	316.5	127	8.5	103.4	3.525	0.018	0.918	0.147	4.303	531.5	0.048	0.903	1.51	61.886	18.858	315.9	92.9
12	B	2573	E1m2	mudstone	264.3	120.7	4.3	15.3	2.582	0.019	0.739	0.2	4.132	463.7	0.057	0.819	1.69	65.877	17.17	273.6	112.8
13	B	2575	E1m2	mudstone	250.5	122.9	2.4	16	3.542	0.038	0.68	0.374	4.105	437.5	0.056	1.075	1.765	65.446	16.716	269.3	96.5
14	B	2576	E1m2	mudstone	326	175.8	6.3	60.8	3.648	0.031	1.047	0.476	4.529	613	0.07	1.197	1.826	62.113	19.057	300.7	166.9
15	B	2586	E1m2	mudstone	271.1	137.2	8.2	73	2.525	0.019	0.719	0.187	4.419	824.6	0.058	0.828	1.723	66.749	16.555	263.8	79.5
16	B	2586.7	E1m2	mudstone	253.8	120.6	11.9	110.3	13.942	0.212	0.677	0.581	3.482	526.3	0.056	1.786	1.356	52.153	14.118	265.3	148.1
17	B	2587.1	E1m2	mudstone	217.2	121.6	8.8	105.1	3.606	0.035	0.756	0.275	4.624	530.1	0.053	1.139	1.494	62.051	18.808	297.8	96.7
18	B	2587.6	E1m2	mudstone	294.8	123	2.2	126.5	2.888	0.023	0.604	0.284	4.173	549.6	0.05	0.745	1.838	68.724	14.674	254.7	83.4
19	C	2742	E1l1	mudstone	252.1	162.1	9.1	181.3	4.375	0.02	0.741	0.174	4.388	581	0.056	0.73	1.602	63.995	16.792	313.5	85.1
20	C	2745	E1l1	mudstone	371.3	155.3	1.1	89.5	4.182	0.023	0.813	0.211	3.91	554.8	0.056	0.704	1.747	62.787	15.35	255.5	72.3
21	D	3333	E1l2	mudstone	262.8	184.6	11.4	197.3	7.86	0.072	1.012	0.181	6.652	542.7	0.081	1.549	0.77	48.144	24.023	407.7	63.3
22	D	3339	E1l2	mudstone	290.2	205.2	3.5	192.9	4.563	0.054	0.817	0.202	4.454	607.7	0.065	0.796	1.787	64.201	15.352	268.9	62.9
23	D	3348	E1l2	mudstone	283.7	177.5	7.4	96.9	2.929	0.022	0.606	0.165	4.708	678.2	0.054	0.584	2.003	68.274	15.449	286.7	79
24	D	3349	E1l2	mudstone	464.2	236.1	5.6	105.5	3.513	0.015	1.195	0.13	5.284	538.1	0.088	0.804	1.796	59.876	19.213	344.3	71.4
25	D	3756	E1y	mudstone	253.2	143.5	1	107.8	3.205	0.017	1.001	0.133	6.486	555.1	0.069	1.187	1.348	56.775	22.399	410	75.9
26	D	3758	E1y	mudstone	271.1	153.5	2	104	3.018	0.018	0.72	0.16	5.468	647	0.069	0.849	1.735	65.299	17.824	318.8	97.8
27	D	3759	E1y	mudstone	265.9	177.6	5.7	111.1	4.64	0.025	0.91	0.226	5.584	666	0.078	1.155	1.675	59.771	19.056	332.3	95.6
28	D	3760	E1y	mudstone	329.3	166.7	8.1	75.2	4.302	0.04	0.891	0.134	6.242	597.1	0.077	0.94	1.632	57.021	21.348	361.7	79.7

.

3.2. Methods

The instrument used in this experiment was an AXIOSMAX X-ray fluorescence spectrometer produced by Panalytical, the Netherlands. It uses a rhodium target to test major and trace elements such as Fe, Si, Al, P, Ca, Cu, Ba, Cr, Zr, V, and Ni.

According to the characteristics of the samples and research needs, twenty-eight mudstone samples were selected for major and trace element tests. Samples were strictly pretreated and tested before testing in order to prevent contamination and ensure objectivity.

The pretreatment process of the samples included putting the samples into an incubator (60 °C), taking them out 24 h later, and then sampling and crushing them to a 200 mesh. After packaging, the samples were taken to the State Key Laboratory of Marine Geology of Tongji University for testing. There were three parts in the test process: weighing, fusing, and testing (Figure 6). The steps were as follows. First, the powder samples were dried again for 2 h; second, the fluorescent agent was heated at 600 °C for 2 h, cooled in the oven, and poured into the original bottle marked with the date; third, a total of 7.0000 g of fluorescent agent was weighed and fully mixed with the 0.70000 g samples; the mixed samples were then poured into a clean 50 mL Pt crucible, two drops of 20 g/50 mL lithium bromide and two drops of 30% hydrogen peroxide were added, respectively, and melted them in the fusion machine at 1050 °C for 8 min. After hearing a beep, we let the Pt plate oscillate for 4 min in a swinging working state, and waited for the instrument to enter the “standing” state. After standing, the crucible was taken out and poured the sample evenly into the Pt plate for standing, and cooled it in front of the furnace to prevent sudden cooling. Finally, the cooled melt was put into the X-ray fluorescence spectrometer for testing. It should be mentioned that in this experiment, one standard sample was added to every 14 test samples as a control. The accuracy of the experimental data was verified by comparing the test values as well as the standard values of the standard sample. After calculation and analysis, the relative error of the major and trace elements was less than 2% and less than 10%, respectively. The experimental data were accurate and satisfied the analytical requirements.

4. Results

4.1. Geochemical Characteristics of Elements

4.1.1. Enrichment Characteristics of Elements

We selected the representative major and trace elements to present in Figure 7. It can be seen that the trace elements Fe, Na, and Al in the Paleocene generally showed obvious enrichment in micronutrients, with contents of 21,880~139,420 μg/g, 7700~20,030 μg/g, and 138,620~240,230 μg/g, respectively. The trace elements Sr, Cu, Mn, Rb, and Zn were relatively deficient, with contents of 120.6~537.7 μg/g, 1.0~11.9 μg/g, 140~2120 μg/g, 220.4~410 μg/g and 62.9~166.9 μg/g, respectively. Moreover, there was little difference in the changing trend between E₁m², E₁l¹, E₁l², and E₁y (

Table 2. The distribution of the major and trace elements of the Paleocene in the West Lishui Sag.

Elements	E1m2	E1l1	E1l2	E1y
Fe	21,880~139,420 39,322.2(18)	41,820~43,750 42,785(2)	29,290~78,600 47,162.5(4)	30,180~46,400 37,912.5(4)
Na	10,830~18,380 14,628.9(18)	16,020~17,470 16,745(2)	7700~20,030 15,890(4)	13,480~17,350 15,975(4)
Al	138,620~192,840 170,527.8(18)	153,500~167,920 160,710(2)	153520~240230 185,092.5(4)	178,240~223,990 201,567.5(4)
Sr	120.6~537.7 193.1(18)	155.3~162.1 158.7(2)	177.5~236.1 200.9(4)	143.5~177.6 160.3(4)
Cu	2.2~11.9 7.2(18)	1.1~9.1 5.1(2)	3.5~11.4 6.98(4)	1.0~8.1 4.2(4)
Mn	140~2120 327.2(18)	200~230 215(2)	150~720 407.5(4)	70~400 250(4)
Rb	220.4~352.4 271.3(18)	220.4~352.4 271.3(2)	255.5~313.5 284.5(4)	208.9~407.7 326.9(4)
Zn	79.5~166.9 102.1(18)	72.3~85.1 78.7(2)	62.9~79 69.15(4)	75.9~97.8 87.3(4)

(minimum~maximum)/(average value)(number of samples).

).

4.1.2. Correlation of Elements

The correlation of elements is mainly controlled by two factors. On one hand, it is controlled by the geochemical properties of the elements, and the elements with similar chemical properties have a better correlation. On the other hand, it is affected by epigenesis and diagenesis, which make the elements differentiate and the correlation become worse. Through the R-type cluster analysis of elements in the study area, all elements in the study area could be divided into as many as 12 groups, with a correlation coefficient of 10 as the benchmark (Figure 8). Among them, K, Rb, Ti, and Al formed a group. Fe, Mn, and Mg also formed a group, and their correlation was good while the correlation of other elements was poor. The analysis showed that the Paleocene climate in the Lishui Sag was warm, and the chemical weathering and biological weathering were strong, which led to the poor correlation between elements [19]. In addition, the area experienced transgression and regression in the Paleocene, which caused great changes in the sedimentary environment, which may be another important reason for the differentiation of major and trace elements in the study area and the deterioration of the correlation between elements.

4.2. Paleoclimate

Paleoclimate studies can be carried out based on the existence of some characteristic rock markers and paleontological fossils contained in sedimentary rocks as well as geological methods such as paleomagnetism and carbon and oxygen isotopes [20,21,22]. This study mainly used the elemental ratio method to judge the paleoclimate of the Paleocene in the West Lishui Sag. The paleoclimate evolution is closely related to the distribution of elemental content, the changes in the elemental ratio, and the paleosalinity. The distribution of elements in mudstone and other rocks is influenced by the paleoenvironment such as paleoclimate and paleosalinity, to a certain extent. When the paleoclimate changes, the paleowater depth will also rise and fall, which will cause the paleoenvironment to change accordingly and affect the distribution of the elements and the changes in the elemental ratios. The elemental ratio has been widely used as an index for restoring paleoclimate. However, it is necessary to select different ratios of major and trace elements according to the sedimentary environment as the index to judge the paleoclimate.

4.2.1. Cu/Sr

The paleoclimate determines the paleoenvironment fundamentally, which in turn affects the origin of source rocks. Since Sr and Cu are very sensitive to changes in climatic conditions, the Cu/Sr ratio is often used to reconstruct the paleoclimatic conditions. When Cu/Sr > 0.05, it indicates a humid climate, and vice versa, it reflects a dry climate [23,24].

The Cu/Sr ratios of the Paleocene in the West Lishui Sag ranged from 0.007 to 0.099, with an average of 0.039. The Cu/Sr ratios of the E₁y were 0.049~0.007, with an average of 0.025, and the ratios decreased upward. The Cu/Sr ratios of the E₁l were0.007~0.062, with an average of 0.03. The Cu/Sr ratios of the E₁m² were 0.015~0.099, with an average of 0.044. From the changing trend, the paleoclimate of the E₁y and E₁m was relatively wetter than that of the E₁l, and the E₁l was in an extremely dry stage. The West Depression of Lishui Sag was in a dry climate during the Paleocene as a whole (Figure 9).

4.2.2. Mg/Ca

Like Cu/Sr, Mg/Ca can also indicate the variation in paleoclimate. Generally, when the Mg/Ca ratio is high, it indicates a dry climate, and when it is low, it indicates a humid climate, but the extremely dry alkali layer is just the opposite [25]. The contents of K and Na in the West Lishui Sag are high (Table 1, Figure 7), which is likely to participate in precipitation to form the alkali layer. By comparing the changing trends of Mg/Ca and Cu/Sr, it is evident that the two trends are consistent, further demonstrating the extreme dry climate conditions (Figure 9).

4.2.3. CaO/(MgO × Al₂O₃)

The ratio of CaO/(MgO × Al₂O₃) can reflect the relative level of the content of calcium carbonate, which can indicate the changes in temperature. A value greater than 0.01 indicates a warm period, while a value less than 0.01 indicates a relatively cold period [26].

The CaO/(MgO × Al₂O₃) ratios of 28 samples of the Paleocene in the West Lishui Sag were 0.005~0.03, with an average of 0.021 (Figure 9). The CaO/(MgO × Al₂O₃) ratios of the E₁y were 0.005~0.011, with an average of 0.008. The CaO/(MgO × Al₂O₃) ratios of the E₁l were 0.005~0.02, with an average of 0.014. The CaO/(MgO × Al₂O₃) ratios of the E₁m² were 0.007~0.103, with an average of 0.026. However, the CaO/(MgO × Al₂O₃) ratios in the lower part of the E₁m² from 2573 m to 2587.6 m were all greater than 0.01, with an average of 0.018, indicating that it was in a warm climate. The ratio of the upper part of the E₁m² ranged from 0.007 to 0.1, with an average of 0.032. By comparing the CaO/(MgO × Al₂O₃) ratio of three layers, it can be found that the temperature of the West Depression increased slightly from the E₁y to the E₁l. The temperature dropped slightly from the E₁l to the lower part of the E₁m², and then to the upper part of the E₁m²; the temperature increased rapidly and fluctuated in multiple periods, locally showing a relatively cold period. Through the comparative analysis of the changing trend of the ratios of various elements (Figure 9), it was considered that the conclusions drawn by the elemental ratio above were consistent. The West Depression of Lishui Sag was in a dry climate during the Paleocene as a whole and experienced the evolutionary process of periodic exchange of humidity and dryness from the E₁y to the E₁m.

4.3. Paleotemperature

There are many methods to reconstruct the palaeotemperature such as the oxygen isotope method and trace element method, and so on [26]. This study used the above two methods to reconstruct the paleotemperature of the Paleocene in the West Lishui Sag, East China Sea Basin.

4.3.1. Sr Element Calculation of Paleotemperature

The paleotemperature was calculated in combination with the test data of the China University of Geosciences by using the empirical formula T = (2578 − Sr)/80.8 °C [25,26,27]. The T values of 41 samples in the E₁m² were 25.25 °C~31.41 °C, with an average of 30.14 °C. The T values of 48 samples in the E₁l¹ were 29.43 °C~31.16 °C, with an average of 30.54 °C. The T values of 21 samples in the E₁l² were 28.98 °C~30.67 °C, with an average of 30.21 °C. The T values of four samples in the E₁y were 29.71 °C~30.13 °C, with an average of 29.92 °C (Figure 10). Compared with the E₁l² and E₁m, the E₁y was in a relatively cold period, which is consistent with the above CaO/(MgO) × Al₂O₃).

The empirical formula does not take into account that there was a small amount of Sr in atmospheric precipitation and the loss of Sr after later deposition, resulting in lower Sr content and higher calculated paleotemperature. However, it can still be seen that it was in a warm climate and the sedimentary environment was relatively stable.

4.3.2. Oxygen Isotope Calculation of Paleotemperature

Twenty-three sandstone carbonate cement samples from the sandstone reservoirs of four wells in the West Lishui Sag were tested for whole-rock carbon and oxygen stable isotopes (

Table 3. The test data of the whole rock carbon and oxygen stable isotopes of sandstone in the West Lishui Sag (according to Shanghai Branch Company of CNOOC).

Well	Formation	Depth (m)	δ13CPDB	δ18OPDB	T (℃)
A	Upper part of E1m2	2235.4	−4.008	−10.539	66.140
		2244.4	−3.862	−9.981	63.149
		2247.9	−2.377	−10.223	64.443
		2249.9	−2.278	−10.715	67.089
		2291.5	−1.323	−10.768	67.375
		2293.5	−1.162	−10.836	67.742
B	Lower part of E1m2	2575.74	−4.608	−10.665	66.819
		2577.2	−2.488	−11.788	72.926
		2580.35	−3.05	−11.596	71.875
		2581.3	−2.273	−9.689	61.593
		2581.7	−4.355	−10.103	63.800
		2582.93	−2.114	−10.986	68.554
		2583.63	−2.037	−10.951	68.365
		2585.3	−2.022	−9.825	62.317
		2588.7	−7.508	−10.397	65.376
C	E1l1	2741.2	−2.841	−8.651	56.119
		2744.94	−3.512	−8.726	56.512
		2748.07	−3.264	−8.812	56.963
D	E1l2	3332	−4.944	−15.158	91.858
		3344.45	−4.545	−16.307	98.521
		3344.84	−5.655	−16.794	101.376
		3347	−4.986	−15.132	91.708
		3347.5	−5.18	−15.255	92.416

). We used oxygen isotope values to calculate the paleotemperature with the following formula:

T = 13.85 − 4.54 × δ¹⁸O + 0.04 × (δ¹⁸O)²

(1)

The paleotemperature ranged from 56.12 °C to 101.37 °C, with an average of 71.44 °C, which was significantly higher than the values calculated by Sr (Figure 11). This is because oxygen isotopes exchanged strongly after late diagenesis.

Previous studies have shown that oxygen isotopes are sensitive to changes in the environment after the sedimentary period. It is generally believed that when δ¹⁸O_PDB is less than −10‰, the diagenesis is strong, and the carbon and oxygen isotopes change strongly. When δ¹⁸O_PDB is less than −5‰, the rock has undergone a certain diagenesis, but the composition and content of carbon and oxygen isotopes change slightly [28,29,30]. The δ¹⁸O_PDB values of the samples measured in the West Lishui Sag were generally less than −10‰, with an average of −11.47‰. There are two reasons that caused the oxygen isotope values to be more negative than that of seawater in the same period. First, atmospheric fresh water can lower the oxygen isotope. Second, because the temperature during the burial period is higher, the heavier oxygen isotopes enter the fluid, and the lighter oxygen isotopes enter the reservoir rock, resulting in a negative oxygen isotope, which reduces the significance of the oxygen isotope as an indicator of paleotemperature during the sedimentary period, and this mainly reflects the diagenetic temperature during diagenesis [31,32].

Combined with the calculation results of the Sr and oxygen isotope, it is considered that the climate was warm and the sedimentary environment was relatively stable during the sedimentary period of the Paleocene in the West Depression of Lishui Sag.

4.4. Offshore Distance and Paleobathymetric

Paleobathymetrics can be comprehensively judged by lithology, sedimentary structure, and biological remains. Meanwhile, modern sedimentary elemental geochemistry research shows that due to the differentiation in elements during sedimentation, the accumulation and dispersion of elements has a certain relationship with the offshore distance, which can also be used to determine paleobathymetry [19,30,31].

4.4.1. Determination of Paleobathymetrics by Sedimentological Markers

The distribution and lithology of sediments are related to the paleobathymetrics. Generally speaking, when the sedimentary water body changes from shallow to deep, the content of sand in the sediment will gradually decrease and the content of clay will increase accordingly. Sedimentary structure is a good indicator of the water depth and hydrodynamics. Shallow water areas have a variety of beddings including large-scale cross-bedding, ripple marks, and erosion. If the sediment is exposed to the surface, dry cracks, rain-print, and trickle marks are common bedding structures. Fine horizontal bedding is mainly formed in deep water areas, with continuous rhythmic developed. In sedimentary environments lacking remains fossils, relative paleobathymetry can be determined by relic fossils such as burrows, footprints, crawl marks, and other bioturbation [3].

The E₁y was mainly composed of gray and gray-black mudstone, intercalated with thin layers of fine sandstones. The flamy structure was developed in the layer (Figure 12a), which indicates that it is in deep lacustrine deposits and is weakly affected by terrigenous inputs; the E₁l² was mainly dominated by gray-white fine sandstone with a small amount of gray mudstones. Tabular bedding (Figure 12b) and bioturbation (Figure 12c) were developed in the layers, indicating that it is in an oxygenated near-shelf environment with relatively shallow water bodies and sandy deposition, and the sedimentary hydrodynamics are strong. The E₁l¹ was mainly dominated by gray and gray-black thick mudstone, intercalated with silty mudstone and argillaceous siltstone in the middle. Flamy structures and worm burrows could be seen in the layers (Figure 12d), indicating that the water body varied in depth. The E₁m² was mainly dominated by gray massive fine sandstone and black mudstone, with thin interbeds of grey fine sandstone and mudstone (Figure 12e), and thin coal seams developed locally. In the E₁m², wavy cross-bedding (Figure 12f), parallel bedding (Figure 12g), and carbonized plant fragments (Figure 12h) could be observed. The Bauma sequence (Figure 12i), mud, and gravel as well as a large amount of carbon debris could be seen in the upper part of the E₁m², indicating that the water body gradually became deeper from the bottom to top. This is consistent with the changes in sea level (Figure 5).

4.4.2. Elemental Ratio Method

Zr is a typical terrestrial element. The farther away from the terrigenous province, the lower the content in the rock. However, the distribution of Zr in sedimentary rocks is dominated by Al, so the Zr/Al value can represent the terrigenous components transported in short distances and the changes in the paleobathymetrics. The smaller the value, the farther the offshore distance and the deeper the water body. The migration and enrichment of Rb and K in water are closely related to clay, and Rb is more easily adsorbed by clay than K. The value of Rb/K is commonly used to indicate changes in the paleobathymetrics, and the larger the value, the deeper the water body [33]. Therefore, F = Rb × Al/(Zr × K) can be used to reconstruct the paleobathymetrics. The larger the value, the deeper the water body [34].

The analysis results show that F values in the West Lishui Sag ranged from 2.70 to 7.50, with an average of 4.41 (Figure 13). The F values of the E₁y were 3.76~5.60, with an average of 4.36. The F values of the E₁l were 2.70~5.60, with an average of 3.50. The F values of the E₁m² were 3.04~7.50, with an average of 4.66. According to the analysis of the calculation results of F, from the E₁y to the lower part of the E₁m², the F value increased first, then decreased, and then increased again, indicating that the paleobathymetrics shows a trend of getting deeper first, then shallower, and then deeper on the whole. Combined with the analysis results of the paleobathymetric and core observations, it is considered that the E₁y in the sampling section was a lacustrine deposition with a gradually deepening water body. The E₁l² was a shallow marine deposition with a large set of gray-white sandstone developed. In the E₁l¹, transgression occurred and the water body gradually deepened. The E₁m² developed a descending system tract accompanied by multiple transgression and regression cycles. The upper part of the E₁m² experienced transgression, and the water body gradually became deeper. At the initial stage of sedimentation of the E₁m¹, the sea water range continued to expand, showing an open littoral-neritic sedimentary environment, and the Lingfeng Uplift was submerged. At the end of the sedimentary stage of the E₁m¹, the transgression ended and the large-scale seawater subsided (Figure 5).

4.5. Paleoredox Environment

Argillaceous sediments contain Cu, Zn, V, Ni and other environmentally sensitive elements, and their enrichment is often controlled by the redox of seawater [35,36,37,38,39]. Therefore, using the content and ratio of elements sensitive to the environment in argillaceous sediments can restore the redox conditions of the paleosedimentary environment and judge the redox property.

4.5.1. Cu/Zn

Fe²⁺/Fe³⁺ is generally used as a geochemical indicator of oxygen fugacity in the sedimentary environment for muddy sediments, but its ratio changes greatly after undergoing certain geological processes, which is difficult to objectively reflect the oxidation–reduction situation of the original sedimentary environment, especially in the study of paleostratigraphy. Cu and Zn are copper group elements, which can be separated during the sedimentary process due to the differences in the medium oxygen fugacity, forming a sedimentary zone of transition from Cu to Zn with the decrease in medium oxygen fugacity. Therefore, the Cu/Zn ratio changes with the change in the medium oxygen fugacity. In the geological environment without a large-scale igneous process and advanced metamorphism, the Cu/Zn ratio is stable, which can be used as an indicator to reconstruct the paleoredox environment [40,41,42]. Generally speaking, when the Cu/Zn ratio is greater than 0.63, it indicates the oxidation environment. When the Cu/Zn ratio is 0.5~0.63, it indicates a weak oxidation environment. When the Cu/Zn ratio ranges from 0.38 to 0.5, it indicates a reducing-oxidation environment. When the ratio of Cu/Zn ranges from 0.21 to 0.38, it indicates a weak reducing environment. When the ratio of Cu/Zn is less than 0.21, it indicates a reducing environment [42,43].

In the 28 groups of data tested, the Cu/Zn ratios were all less than 0.2 and the maximum value was 0.18, which indicate reducing environments (Figure 14). The average of Cu/Zn in the E₁m² was 0.0743, the average of Cu/Zn in the E₁l was 0.0846, and the average of Cu/Zn in the E₁y was 0.049. The Cu/Zn ratio in the E₁m² appears cyclic, which may be related to the rise and fall in the sea level caused by frequent transgression and regression (Figure 5). Compared with other formations, E₁y had the strongest reducibility, which is more conducive to the preservation of organic matter and the formation of high-quality source rocks.

4.5.2. V/Ni and V/(V + Ni)

V and Ni belong to the iron group of elements, and their ionic valence is easy to change with the degree of oxidation. V and Ni in water are mainly absorbed and precipitated by colloidal particles or clay. However, V is more likely to be enriched in an oxidation environment, while Ni is more likely to be adsorbed in a reducing environment. Therefore, V/Ni and V/(V + Ni) are widely used to evaluate the redox conditions of water bodies during sediment formation [43,44]. Generally speaking, a V/Ni ratio less than 1 indicates an oxidation environment, while a V/Ni ratio greater than 1 indicates a reducing environment. A high V/(V + Ni) ratio (>0.84) reflects the stratification in the water body and the anaerobic environment where H₂S appears at the bottom. A medium ratio (0.60~0.84) reflects an anaerobic environment where the water stratification is not strong. When the ratio is low (0.46~0.60), it indicates an oxygen-poor environment with weak water stratification.

Due to the lack of V in this test, the data of V and Ni of the same interval tested by the China University of Geosciences (2017) provided by the CNOOC Shanghai branch are cited for comparative analysis (

Table 4. The test data of V and Ni in the West Lishui Sag (according to China University of Geosciences).

Number	Well	Depth (m)	Formation	Ni (ppm)	V/%	V/Ni	V/(V + Ni)
1	B	2580.73	E1m2	0.01	0.024	2.400	0.706
2	B	2580.79	E1m2	0.013	0.027	2.077	0.675
3	B	2580.8	E1m2	0.014	0.036	2.571	0.720
4	B	2580.85	E1m2	0.013	0.024	1.846	0.649
5	B	2580.9	E1m2	0.014	0.026	1.857	0.650
6	B	2582.39	E1m2	0.018	0.018	1.000	0.500
7	B	2582.41	E1m2	0.017	0.062	3.647	0.785
8	B	2582.42	E1m2	0.012	0.031	2.583	0.721
9	B	2582.43	E1m2	0.015	0.032	2.133	0.681
10	B	2582.51	E1m2	0.013	0.032	2.462	0.711
11	B	2582.53	E1m2	0.015	0.025	1.667	0.625
12	B	2582.55	E1m2	0.013	0.03	2.308	0.698
13	B	2582.57	E1m2	0.011	0.04	3.636	0.784
14	C	2747.49	E1l1	0.011	0.012	1.091	0.522
15	C	2747.51	E1l1	0.019	0.015	0.789	0.441
16	C	2747.53	E1l1	0.011	0.012	1.091	0.522
17	C	2747.55	E1l1	0.014	0.014	1.000	0.500
18	C	2747.57	E1l1	0.013	0.023	1.769	0.639
19	C	2747.63	E1l1	0.015	0.013	0.867	0.464
20	C	2747.67	E1l1	0.016	0.013	0.813	0.448
21	C	2747.8	E1l1	0.021	0.019	0.905	0.475
22	C	2747.82	E1l1	0.014	0.021	1.500	0.600
23	C	2747.84	E1l1	0.019	0.02	1.053	0.513
24	C	2747.86	E1l1	0.011	0.018	1.636	0.621
25	C	2747.9	E1l1	0.013	0.013	1.000	0.500
26	C	2747.94	E1l1	0.017	0.014	0.824	0.452
27	C	2747.99	E1l1	0.018	0.015	0.833	0.455
28	C	2748.03	E1l1	0.014	0.012	0.857	0.462
29	C	2748.05	E1l1	0.013	0.014	1.077	0.519
30	C	2748.08	E1l1	0.016	0.012	0.750	0.429
31	C	2748.11	E1l1	0.015	0.021	1.400	0.583
32	C	2748.17	E1l1	0.015	0.014	0.933	0.483
33	C	2748.22	E1l1	0.016	0.011	0.688	0.407
34	C	2748.25	E1l1	0.014	0.016	1.143	0.533
35	C	2748.32	E1l1	0.012	0.013	1.083	0.520
36	D	3346.25	E1l2	0.016	0.019	1.188	0.543
37	D	3346.5	E1l2	0.011	0.023	2.091	0.676
38	D	3346.7	E1l2	0.011	0.019	1.727	0.633
39	D	3346.8	E1l2	0.013	0.019	1.462	0.594
40	D	3346.95	E1l2	0.015	0.027	1.800	0.643

).

The average V/Ni ratio of the Paleocene in the West Lishui Sag was 1.54, of which those greater than 1 accounted for 75% of the total data. The V/Ni ratios of the E₁m² were 1~3.64, with an average of 2.32, and the V/(V + Ni) ratios were 0.5~0.78, with an average of 0.68. The V/Ni ratios of the E₁l¹ were 0.69~1.79, with an average of 1.05, which generally indicates a reducing environment. The ratios less than 1 may be caused by regression or terrigenous inputs. The V/(V + Ni) ratios were 0.41~0.64, with an average of 0.50, indicating that the sedimentary water body of the E₁l¹ was in an oxygen-poor environment with weak water stratification. The V/Ni ratios of the E₁l² were 1.19~2.09, with an average of 1.65, indicating a reducing environment. The V/(V + Ni) ratios were 0.54~0.68, with an average of 0.62, indicating that the sedimentary water body of the E₁l² was in an anaerobic environment with weak water stratification (Figure 15).

Compared with the V/Ni and V/(V + Ni) ratios of the E₁m², the V/Ni ratios of the E₁l¹ and the E₁l² were small, indicating that their reducibility was weaker than that of the E₁m², which is consistent with the conclusion of Cu/Zn. The fluctuation of V/Ni ratios in the E₁m² is speculated to be related to the frequent transgression and regression during this period (Figure 5). On the whole, the V/Ni and V/(V + Ni) ratios indicate that the Paleocene in the West Lishui Sag was in a relatively strong reducing environment.

Based on the analysis results of Cu/Zn, V/Ni, V/(V + Ni), and the paleobathymetry, it is considered that the West Depression of Lishui Sag was in a reducing environment during the Paleocene as a whole. The E₁y was the most reductive, with deep paleobathymetry and anoxic at the lake bottom, which was conducive to the accumulation and preservation of the hydrogen-rich organic matter caused by phytoplankton. The water body of the E₁l in the sampling section was relatively shallow and mainly formed in an oxygenated near-shelf environment of sandy deposition. The sedimentary hydrodynamics was strong, the bottom of the water body was oxygenated, and its reducibility was relatively weak, which was not conducive to the accumulation and preservation of hydrogen-rich organic matter. In the E₁m², the water body became deeper and the reducibility gradually increased. The periodical change in the elemental ratios may be related to the frequent transgression and regression during this period. Paleoredox characteristics in the West Lishui Sag are consistent with sedimentary evolution (Figure 5).

4.6. Paleosalinity

Paleosalinity is a record of water salinity in ancient sediments, which can be used as important information to analyze the sedimentary characteristics in geological history. There are many methods to distinguish paleosalinity. For example, paleontology, geochemical methods of major and trace elements and the mass spectrometry of saturated hydrocarbons can be used to quantify water salinity [19,20,45,46,47]. In this study, the elemental ratio method was used to analyze the paleosalinity in the West Depression of Lishui Sag.

4.6.1. Rb/K

The content of K is closely related to the content of clay minerals, especially the content of illite. Rb is mainly transported in the state of suspended colloids. Under alkaline reduction conditions, the colloid of Rb is easily adsorbed by clay and organic matter due to the flocculation effect. Therefore, the higher the water salinity, the stronger the adsorption of Rb by clay and organic matter, and the higher the content of the Rb and Rb/K ratio [48]. The Rb/K ratios were generally greater than 0.006 in marine shale, greater than 0.004 in brackish water, and less than 0.004 in freshwater.

The Rb/K ratios of the Paleocene in the West Lishui Sag were 0.0058~0.0081, with an average of 0.0066. All calculated values were greater than 0.004, indicating a brackish to saltwater environment during the Paleocene in the West Lishui Sag (Figure 13). The Rb/K ratios of the E₁y were 0.0058~0.0063, with an average of 0.0059, which indicates a brackish water environment as a whole and a saltwater environment locally. The Rb/K ratios of the E₁l were 0.006~0.0065, with an average of 0.0063, which indicates a saltwater environment. The Rb/K ratios of the E₁m² were 0.006~0.0082, with an average of 0.0068, indicating a saltwater environment.

4.6.2. Sr/Ba

The Sr/Ba ratio is a common index to judge the salinity of water bodies during the sedimentary period. Generally, the solubility of Sr is greater than that of Ba, and the migration ability of Sr is stronger than that of Ba. With the increase in water salinity, Sr and Ba gradually precipitate out of the water in the form of sulfates. BaSO₄ precipitates preferentially, and SrSO₄ precipitates only when salinity reaches a certain high value. Therefore, Sr and Ba generally do not precipitate in a continental freshwater environment, and the Sr/Ba ratio in sediments is very low. After the continental fresh water enters the sea (lake), a part of Ba precipitates preferentially. At this time, there is no Sr precipitation, and the Sr/Ba ratio is also very low. When Sr and Ba continue to migrate deeper to the sea or lake (salinity gradually increases), the content of Ba gradually decreases due to continuous precipitation. Sr begins to precipitate at this time, and the Sr/Ba ratio in sediments will increase significantly. Therefore, it is feasible to judge the salinity of the sedimentary environment by the Sr/Ba ratio [20,49,50,51]. It is generally considered that the Sr/Ba ratio of marine sediments is higher than that of continental sediments.

The Sr values of the Paleocene in the West Lishui Sag were relatively small, which may be due to the strong diagenesis and the influence of atmospheric fresh water in the later period. Based on previous research results and the actual situation of the research area, this paper considered that a Sr/Ba ratio greater than 0.6 indicates a salt water environment, a Sr/Ba ratio between 0.2 and 0.6 indicates a brackish water environment, and a Sr/Ba ratio less than 0.2 indicates aa fresh water environment. The Sr/Ba ratios of the Paleocene in the West Lishui Sag ranged from 0.17 to 1.06, with an average of 0.35. The measured data were generally between 0.2 and 0.6, with only a few data greater than 0.6 (Figure 16). The Sr/Ba ratios of the E₁y were 0.24~0.28, with an average of 0.26. The Sr/Ba ratios of the E₁l were 0.26~0.44, with an average of 0.33. The Sr/Ba ratios in the E₁m² were 0.17~1.06, with an average of 0.35, and the salinity increased gradually from the bottom to top. The Sr/Ba ratios of the seven samples in the lower part of the E₁m² were 0.17~0.28, with an average of 0.24, which indicates a brackish water environment. The Sr/Ba ratios of the 11 samples in the upper part of the E₁m² were 0.23~1.06, with an average of 0.48, which indicates a transitional environment from brackish water to saltwater as a whole. As shown in the changing trend of the Sr/Ba ratios, the E₁y was in a brackish water environment, which corresponded to its deeper water depth. The E₁l² began to transgress, and the continental faulted-lake basin gradually transformed into a marine basin, resulting in a gradual rise in the water salinity. The Sr/Ba ratios indicate a brackish water environment at this time. The water salinity was lower than that of the normal marine sediments, possibly due to the terrigenous input and the influence of atmospheric precipitation or the relatively shallow core location. At the initial stage of the E₁m², regression occurred and the sea level fell (Figure 5). At the same time, affected by atmospheric rain and other factors, the water salinity decreased. The transgression occurred again in the upper part of the E₁m², the sea level rose, and the sea depth became larger, with a transition environment of brackish to saltwater.

Through comprehensive analysis of the above two methods for judging paleosalinity, it was found that the calculated changing trend of paleosalinity was very similar (Figure 16). It was considered that the paleowater was in the transition environment of brackish water to saltwater during the Paleocene sedimentary period in the West Lishui Sag. The E₁y was in a brackish water environment, and the E₁l was in a brackish-saltwater environment. The lower part of the E₁m² was in a brackish water environment, and the upper part of the E₁m² was in a saltwater environment. From the E₁y to the E₁m², the salinity showed a trend of first decreasing and increasing, then decreasing and increasing. The water body gradually transited from brackish water to saltwater, then to brackish water and finally to the saltwater environment. It is speculated that the repeated cyclic changes of salinity are caused by multiple factors such as frequent transgression and regression, terrigenous inputs, atmospheric precipitation, and seawater evaporation.

4.7. Paleoproductivity

Paleoproductivity refers to the rate at which organisms fixed energy in the process of energy cycle in geological history. Paleoenvironmental factors such as paleoproductivity and organic matter preservation conditions in the basin comprehensively control the formation of high-quality source rocks. The TOC, major and trace elements, isotopes, paleontology, and other related indicators have been used to qualitatively restore the paleoproductivity of the lake basin [35]. Based on the experimental and collected data, this paper used the elemental ratio method and formula method to quantitatively analyze the changes of paleoproductivity of the Paleocene in the West Lishui Sag.

4.7.1. Elemental Ratio Method

Tribocillard et al. believe that the formation of authigenic barite in the surface water is related to biological action, but the specific process is uncertain [23,49,52]. Dehairs et al. believe that it might be related to the decomposition of plankton, which actively or passively absorbs Ba during the growth process of plankton [53]. Ba is supersaturated in water bodies due to the decomposition of plankton, and it forms precipitation when it combines with SO₄²⁻, which is recorded by the sediment. Therefore, the abundance of Ba in the strata can reflect the paleoproductivity of sedimentary period to a certain extent. It has been proven that Ba has a positive impact on the surface of the water and the accumulation of the organic matters, which has been widely used in the evaluation of the paleoproductivity [54]. P is the most important nutrient element of plankton, and it is also positively correlated with paleoproductivity. In order to eliminate the influence of sedimentary organic matter and terrigenous detritus, Ba/Al and P/Ti ratios are calculated to quantitatively evaluate the paleoproductivity of the Paleocene in the West Lishui Sag.

The Ba/Al ratios of the Paleocene in the West Lishui Sag range from 0.0020 to 0.0050, with an average of 0.0032. The Ba/Al ratios of the E₁y were 0.0025~0.0036, with an average of 0.0031. The Ba/Al ratios of the E₁l were 0.0023~0.0044, with an average of 0.0034. The Ba/Al ratios of the E₁m² were 0.0020~0.0050, with an average of 0.0032. The Ba/Al ratios changed little in the West Lishui Sag. The P/Ti ratios of the Paleocene in the West Lishui Sag ranged from 0.052 to 0.169, with an average of 0.082. The P/Ti ratios of the E₁y were 0.069~0.089, with an average of 0.084. The P/Ti ratios of the E₁l were 0.069~0.089, with an average of 0.078. The P/Ti ratios of the E₁m² were 0.052~0.169, with an average of 0.083. The changing trends in the Ba/Al and P/Ti ratios were roughly the same. In the E₁y and E₁l, the paleoproductivity had undergone a process of increasing first and then decreasing. In the E₁m², the paleoproductivity had undergone a cyclic process, and the maximum of the paleoproductivity was between 2258 m and 2293 m (Figure 17). In general, the paleoproductivity of the Paleocene in the West Lishui Sag was relatively low.

4.7.2. Formula Method

The sources of Ba are mainly terrigenous input and biogenesis, and only the biogenesis can reflect paleoproductivity. Therefore, the terrestrial Ba in the samples can be deducted from Ti mainly from the terrestrial input with low solubility in water. The calculation formula is as follows:

X_XS = X_total − Ti_total × (X/Ti)_PAAS

In the formula, X_XS represents the corrected content of a biogenic element; X_total represents the total content of elements actually measured; Ti_total represents the total content of Ti in the measured samples; and (X/Ti)_PAAS represents the element that needs to be corrected in Neoarchean Australian shale. According to Taylor and Mclennan, the PAAS values of Ba, Zn, and Ti were 650 μg/g, 85 μg/g, and 6000 μg/g, respectively. If X_XS is positive, it indicates that the element is marine authigenic enrichment or volcanic hydrothermal enrichment relative to PAAS. If X_XS is negative, it indicates that the element in the sample is mainly contributed by terrestrial materials [26,55].

The Ba_XS values of the Paleocene in the West Lishui Sag ranged from −756.48 to 45.68, with an average of −299.69. The Ba_XS values of the E₁y were −529.32~−133, with an average of −337.58. The Ba_XS values of the E₁l were −756.48~21.7, with an average of −378.35. The Ba_XS values of the E₁m² were −598.32~45.68, with an average of −271.01. The Zn_XS values of the Paleocene in the West Lishui Sag ranged from −97.89 to 52.19, with an average of −18.19. The Zn_XS values of the E₁y were −65.91~−4.2, with an average of −37.49. The Zn_XS values of the E₁l were −97.89~−6.85, with an average of −56.11. The Zn_XS values of the E₁m² were −74.81~52.19, with an average of −4.14.

Comparing and analyzing the Ba_XS and Zn_XS calculations, it was found that only two Ba_XS values were greater than 0 and only eight Zn_XS values were greater than 0, which were all located in the E₁m², while the rest were negative. Generally, when the Ba_XS value is 1000 μg/g~5000 μg/g, the sedimentary environment has high productivity. When the Ba_XS value is 200 μg/g~1000 μg/g, the sedimentary environment has medium productivity, and when the Ba_XS value is 0 μg/g~200 μg/g, the sedimentary environment has low productivity. Therefore, the paleoproductivity of the Paleocene in the West Lishui Sag was low. The organic matter was difficult to preserve, and the hydrocarbon generation potential was poor. Only the E₁m² had a certain hydrocarbon generation capacity. The nutrient elements in the samples mainly came from terrigenous input, rather than biogenesis. Compared with the E₁y and the E₁l, the paleoproductivity of the E₁m² was significantly greater.

By comparing the paleoproductivity, paleobathymetrics, and paleoclimate of the Paleocene in the West Lishui Sag, it was found that the paleoproductivity was well-correlated with paleobathymetry and paleoclimate. Paleoproductivity was positively correlated with the paleoclimate and paleobathymetry (Figure 18 and Figure 19). A warm and humid climate and a deep water body are conducive to the accumulation of paleoproductivity.

4.8. Paleoterrigenous Input

The input of terrigenous sediments can not only provide higher plant debris, but also dilute the biological supply capacity of the lake basin. The elements mainly derived from terrigenous sources (Al, Si, Ti, Zr, etc.) can effectively indicate the content of terrigenous debris in sediments [35]. In this paper, the Si/Al ratio was used to analyze the terrigenous influence on the Paleocene reservoir in the West Lishui Sag.

The Si/Al ratios of the Paleocene in the West Lishui Sag ranged from 2.00 to 5.01, with an average of 3.63. The Si/Al ratios of the E₁y were 2.53~3.66, with an average of 3.00. The Si/Al ratios of the E₁l were 2.00~4.42, with an average of 3.56. The Si/Al ratios of the E₁m² were 2.96~5.01, with an average of 3.79. The changes in the Si/Al ratio had the same trend as that of the paleoproductivity (Figure 20), which also proves that the paleoproductivity of the Paleocene in the West Lishui Sag was greatly affected by terrigenous input from the side. Generally speaking, the characteristics of terrigenous intrusion from the E₁y to the E₁m² were gradually enhanced.

5. Discussion

Based on the above analysis and previous research results, this paper established a paleoenvironment evolution model during the Paleocene sedimentary period in the West Lishui Sag, East China Sea Basin (Figure 21). During the transgressive system tract sedimentary period of the E1y, the climate was warm, the lake level rose, and the lake basin expanded. As a whole, it was a reducing environment of fresh water-brackish water deposition. The reducibility and the paleoproductivity gradually increased. During the sedimentary period of the E₁l², the sea water rapidly invaded from the south and the east of Lishui Sag affected by the rise in the global sea level, which gradually transformed the sag from a faulted-lake basin to a marine basin. In this period, the terrigenous material was relatively few, the climate was dry, and the salinity of the water increased. The sedimentary water body was in an anaerobic reduction environment with weak water stratification, and the reducibility gradually decreased over time. The second transgression occurred during the sedimentary period of the E₁l¹, and the depression scope expanded further. At this time, the climate was relatively cold and dry, the water reducibility decreased, the organic matter in the sediment was relatively low, and the productivity level was insufficient. During the sedimentary period of the descending system tract, the sea level in the depression decreased significantly, and there was a lack of provenance supply in the basin. The early deposition of the E₁m² coincided with the decline in the global sea level, the water body became shallow, the salinity decreased, and the reducibility decreased. At this time, the climate was warm and humid, the sediment supply was sufficient, and the productivity level increased. After that, the sea level entered a slow rise period, during which the water body became deeper, and the salinity and the reducibility increased. In general, the Paleocene in the West Lishui Sag was in a dry reduction environment, and the paleoproductivity level was not high, accompanied by strong terrigenous input. The water salinity was brackish-salt, with little change in water temperature longitudinally.

6. Conclusions

Based on the study of the characteristics of trace elements and their vertical variation of the Paleocene in the West Depression of Lishui Sag, combined with petrological and mineralogical characteristics, sedimentary characteristics, and carbon and oxygen isotopes, the paleoenvironmental characteristics such as paleoclimate, paleotemperature, paleobathymetry, paleosalinity, paleoredox conditions, paleoproductivity, and paleoterrigenous input were analyzed vertically, and the following conclusions were drawn:

(1)

The characteristics of the major and trace elements were similar in the E₁m², E₁l¹, E₁l², and E1y, and the paleoclimate of the Paleocene in the West Depression of Lishui Sag was dry as a whole. From the E₁y to the E₁m, it experienced an evolutionary process of the periodic exchange of humidity and dryness.

(2)

Combined with the Sr and oxygen isotope calculation results, the paleotemperature of the Paleocene in the West Lishui Sag was warm and the sedimentary environment was relatively stable. However, the content of Sr became smaller after later deposition, so the calculated paleowater temperature was higher. In addition, oxygen isotopes were affected by diagenesis, resulting in a negative oxygen isotope value, which needs further research.

(3)

The Paleocene in the West Lishui Sag was mainly in a reducing environment of brackish-salt water with weak water stratification. As a whole, the water depth showed a trend of becoming deeper, then shallower, and then deeper. The upper part of the E₁m² had the deepest water depth, the strongest reducibility, and the highest salinity.

(4)

The values of Baxs, Znxs, and the ratio of trace elements showed that the paleoproductivity of the study area was low, and the hydrocarbon generation potential was poor. The nutrient elements mainly came from the terrigenous input rather than the biological origin. Meanwhile, the Si/Al ratios also showed that the study area was greatly affected by the terrigenous input, and the terrigenous intrusion characteristics gradually increased from the E₁y to the lower part of the E₁m².

(5)

The paleoproductivity was affected by the paleoclimate and paleowater depth. The warm and humid climate and the deep water body were conducive to the accumulation of paleoproductivity.