Determination of Paleocurrent Directions Based on Well Logging Technology Aiming at the Lower Third Member of the Shahejie Formation in the Chezhen Depression and Its Implications

The Bohai Bay basin, mainly formed in the Cenozoic, is an important storehouse of groundwater in the North China Plain. The sedimentary deposits transported by paleocurrents often provided favorable conditions for the enrichment of modern liquid reservoirs. However, due to limited seismic and well logging data, studies focused on the macroscopic directions of paleocurrents are scarce. In this study, we obtained a series of well logging data for the sedimentary layers of Es3L Formation in the Chezhen depression. The results indicate the sources of paleocurrents from the northeast, northwest, and west to a center of subsidence in the northern Chezhen depression at that time. Based on the well testing data, the physical properties of the layers from Es3L Formation in this region were generally poor, but two abnormal overpressure zones were found at 3700–3800 m and 4100–4300 m deep intervals, suggesting potential high-quality underground liquid reservoirs. By combining with other geological evidence, we suggest that the Pacific Plate was retreating and changing its direction from NE–SE to W–E and the Bohai–Luxi block was suffering an extrusion from NE induced by the Lan–Liao and Tan–Lu strike-slip faults in the early Paleogene.


Introduction
The North China Plain has one of the largest groundwater stores in the world. The groundwater reservoirs are widely distributed in the sedimentary interlayers of the post-Cenozoic age in the Bohai Bay basin (Figure 1a), laying in the north of the North China Plain. The Chezhen depression ( Figure 1b) is a representative secondary sub-basin tectonic unit in the Bohai Bay basin [1][2][3]. The study of this depression is of great help to understand the distribution of groundwater reservoirs and the tectonic genesis of the Bohai Bay basin. In previously published papers, many studies have focused on the stratigraphy, sedimentology, microtectonics, oil-gas exploration, and thermal evolution history of the eastern Bohai Bay basin [4][5][6][7][8][9]. However, due to limited seismic and well logging data, studies focused on the macroscopic direction of paleocurrents are scarce, and the coupling relationship between paleocurrents and the tectonic background has not received enough attention [10,11].
The migration of paleocurrents controls the sedimentary filling process and the composition of sediments in the basin, and it further influences the scale and physical properties of underground liquid reservoirs [12][13][14]. In particular, the paleowater flow was always accompanied by mineral transportation and sedimentary screening, leading to the deposition of mature sediments in the downstream area to benefit the modern preservation of groundwater. Therefore, the direction of the provenance of sedimentary deposits is a reliable indicator of the macroscopic direction of paleocurrents [15][16][17]. Furthermore, the state of paleocurrents reflects the paleogeomorphologic characteristics. In general, paleocurrents always flow from higher to lower altitudes, so the paleogeomorphologic features can be restored by using the results of paleocurrents to illustrate the dynamic changes of the surrounding tectonics [18][19][20].
Traditional methods for provenance analysis mainly include heavy mineral analysis, clastic rock analysis, sedimentary analysis, and fission track methods [21][22][23][24][25][26]. However, the experimental results of traditional methods are easily limited by the geographical locations of the sampling sites, the number of samples, and the erosion and chemical weathering of outcrops, leading to inconclusive results. In well logging technology, data such as gammaray and photoelectric absorption data are the sum of the specific information obtained from the target formations buried underground. Therefore, the required data for provenance analysis can be obtained directly from the original rocks [27,28], which greatly improves the accuracy. In recent years, well logging methods have been used to determine the directions of paleocurrents and the sedimentary provenance by quantitative and semi-quantitative calculation, and good results have been achieved [29,30].
In this paper, we obtained the seismic and well logging data from seven wells in the Chezhen depression focusing on the sandstone layers in the lower part of the third member of the Shahejie Formation (Es 3 L ) in the early Paleogene. The directions of paleocurrents and the sedimentary provenance in this area were determined, and then favorable locations of underground liquid reservoirs were predicted. By combining the results with the tectonic background of the Bohai Bay basin in the early Paleogene, we interpreted the geotectonic dynamic of the Chezhen depression and the block it belongs to.  [3]. It should be noted that the logging data from the seven wells (yellow points) were used in most results; however, the reconstruction of the paleogeomorphologic map was performed based on all 40 wells in this region. The locations of the other 33 wells are hidden in this map following privacy policies.

Geological Information
Located in the east of the North China block, the Bohai Bay basin is the central region where the Paleo-Asian Ocean, Tethys Ocean, and Pacific Ocean interacted [32][33][34][35]. It is a large-scale basin where the Mesozoic-Cenozoic rocks are superimposed on the pre-Paleozoic basement of the North China craton [36][37][38][39]. The tectonic evolution of the Bohai Bay basin can be divided into three scenes: the first syn-rift and post-rift scene in the Jurassic (201.3-145.0 Ma); the second syn-rift and post-rift scene in the Cretaceous (145.0-65.0 Ma); and the third syn-rift and post-rift scene from the Paleogene (~65.0 Ma) to the present [40].
Our research was carried out in the Chezhen depression, laying at the southeast of the Bohai Bay basin (Figure 1b) [1]. It is one portion of a series of tectonic combinations of uplifts and depressions arranged in a northwest direction; it is adjacent to the Bohai bay and is bounded by the Tan-Lu strike-slip fault in the east and the Lan-Liao strikeslip fault in the west [41][42][43]. The Chezhen depression is a Cenozoic continental faulted basin developed based on the long-term reconstruction of the cratonic basement [1,3]. It is a synsedimentary half-graben controlled by the Chengnan normal fault that connects with the Chengning uplift and a partially exposed Precambrian crystalline basement to the north, and the Cenozoic sedimentary overlapped cover extends to the Yihezhuang uplift to the south [44]. This pattern of W-E trending fault in the north and stratigraphic overlapping in the south may suggest a crustal stretching. The Chezhen depression can be further divided into three sub-tectonic regions: the steep south-dipping slope formed by the fault in the north, the gentle slope belt extending to the south, and the depression area formed inside [1]. The interior of the Chezhen depression has stable sedimentation and weak faulting, and only four weak secondary shear faults have developed in it. Thus, the Chezhen basin is suitable for drilling and well logging research in the depression.
The Chezhen depression contains the ancient basement in the lower part and the strata of sedimentary cover generated during the three rift cycles in the upper part [1,3]. The basement is composed of a Precambrian metamorphic basement and metamorphic or unmetamorphosed Paleozoic sedimentary rocks, including Cambrian-Ordovician marine carbonates and Carboniferous-Permian clastic sediments and limestones. The cover consists of Jurassic to Cretaceous volcanic rocks and terrestrial sediments, as well as lacustrine and fluvial sediments in the Cenozoic ( Figure 2). The thick Paleogene strata are mainly composed of thick Shahejie Formation (Es) sediments, which are further divided into four members. Es 4 is composed of a 200-1000 m thick red bed, mudstones with interbedded gypsum, and salt rocks. Es 3 consists of 300 m to 1200 m gray-black sandstones and conglomerates. Es 2 includes 100-600 m variegated mudstones and inter-layered siltstones. Es 1 contains 50 m to 400 m sandstones, mudstones, shales, and biogenic limestones [3,45,46]. Among them, the lower sub-member of Es 3 , the target formation in our study, is considered to have the largest high-quality reservoirs in this region, making it of great exploration value.

Methods
Well logging is one of the most commonly used geophysical wireline techniques applied to hydrocarbon exploration and reservoir description [47]. It is widely used for applications including lithotype discrimination [48], estimation of shale content [49,50], lithological correlations from well to well in sediments [51], interpretation of the paleo-climate and/or paleo-environment [52][53][54], sedimentary facies characterization, and reservoir analysis [55]. In this study, the well logging technique was applied as an indicator of paleocurrents and sedimentary provenance directions because the variations of index components from sediments are sensitive to erosion during migration. Where lithological variations and deformations are gentle, the changes in the distribution and maturity of sedimentary deposits and abundance of particular active materials can thus be deemed a corresponding kinestate of the flowage of paleocurrents.
Gamma-ray (GR) logging data are measured from the "total" gamma activity surrounding a borehole. This activity is a sum of radioactive contributions from the decay series of U 238 (mainly from the intermediate daughter Bi 214 ) and Th 232 (mainly from Tl 208 ), and from K 40 [56]. Th mainly exists in heavy/resistate minerals (e.g., thorite, monazite, and rutile) [57] and is generally presumed to be contained by clay minerals in clastic sediments [58,59]. Because Th is mainly transported in a state of solid particles [58,59], the concentration of Th is decreased by hydraulic sorting and recycling of sedimentary environments along the paleocurrents [60]. U is present in uraninite (only in the case of sediments deposited before oxy-atmospheric conversion), allanite, zircon, and monazite in sedimentary rocks; in various rock-forming minerals; and in association with various colloidal and carbonaceous substances in sediments [57]. In the migration of sediments, U may be extracted by chemical weathering, recycling [61], and the pH and Eh of ancient fluvial systems and transport to the reservoirs along paleocurrents [57]. K is contained mainly in K-feldspars and clay minerals. The survival of K-feldspars in a sedimentary environment is a function of the relief and climatic factors that prevailed in the provenance [62]. The process of sedimentary transport from high to low positions decreases the concentration of K in sediments. Because the K-bearing minerals are less stable compared to Th-bearing minerals, K decays faster than Th in a natural sedimentary environment [63]. Additionally, the effective elimination of K-feldspars renders sediments more quartz-rich and mature. Therefore, along the direction of paleocurrents, the Th/U ratio decreases and the Th/K ratio increases.
The photoelectric absorption (Pe) index of the sedimentary rocks can be measured by lithology-density logging. The Pe index can effectively distinguish the typical lithology, and it suffers little influence from the properties and other contents of fluids in the source rocks [64,65]. It is especially sensitive to the total organic carbon content in mudstone [66]. With paleocurrent transport, the argillaceous components decrease and the Pe index decreases. Therefore, the Pe index decreases following the fluxion of paleocurrents.
Through dip logging, the tendency and dip of the sedimentary layers around the wells can be measured. The variations of tendency and dip corrected by tectonic dip angle are classified into five patterns [30,67]. Among them, the blue pattern presents a similar tendency and decreasing of dip angle related to the increase of depth, suggesting a result of the flow of paleocurrents. The paleocurrent direction is determined by the main tendency. Others are named as red, green, yellow, and white patterns, presenting faults, stable sedimentation, overturned strata, and meaninglessness, respectively.
The permeability, porosity, and thickness of reservoirs were derived from seven wells for the sedimentary layers of the Es 3 L Formation. The well testing process was performed to determine the variations of porosity and permeability with depth.

Results
According to the W-E seismic interpretation profile, the deformation of Es 3 L Formation tends to be gentle in this region, and the fault scarp with a large dip only exists near the eastern boundary (Figure 3a). Combining this with the stratigraphic correlation profile derived from the logging data, the Es 3 L Formation can be divided into six sandsets (Figure 3b). Because Sandset 6 was only identified in the easternmost well drilling and there were no comparable ones in other wells, we excluded it and performed the analyses focused on the data from Sandsets 1-5 in this region. The variations of the Th/U, Th/K, and Pe values of Sandsets 1-5 in the Es 3 L Formation obtained by GR logging and lithology-density logging from seven wells in this region are shown in Figure 4 and Table 1. There is a strong similarity in the statistics for the different sandsets from each well. The standard deviations of data from each well are relatively small, and the scope of AVG ± SD covers most of the data (Figure 4). These results indicate the stability of sedimentary layers in the Es 3 L Formation. Thus, we use the averaged Th/U, Th/K, and Pe values derived from five sandsets to get further results.  Table 1. Green shades are intervals of ± standard deviation (SD). The variation of the averaged Th/U ratio of the five sandsets indicates a decline from Wells C0 and C10 in the north to C6 in the south, and from Well C8 in the west to C2 in the east (Figure 5a). The averaged Th/K ratio indicates an increase from Wells C10 and C0 in the northwest to Well C6 in the south, from Well C8 in the west to Well C6, and from Well C2 in the northeast to Well C6 (Figure 5b). The variation of the averaged Pe index is similar to that of Th/U, which indicates a decline from Wells C10 and C0 in the northeast to Well C6 in the south, from Well C8 in the west to Well C6, and from Well C2 in the northeast to Well C6 (Figure 5c).
The dip logging data were mainly obtained from the cross beddings, and the patterns of the stratigraphic dip variations were identified for each sandset from Wells C10, C01, and C61 ( Table 2). The dip angles are generally slight in this region, and the main tendencies are concentrated. In Wells C10, C01, and C61, the main tendencies are directed southeast, southeast/southwest, and southeast/southwest, respectively. Although secondary green and red patterns were recognized in some sandsets, the dominating dip pattern in these wells is the blue one, suggesting a strong correlation of the scouring of paleocurrents in this region.
The porosity and reservoir thickness for Sandsets 1-5 were obtained from each well ( Table 3). Based on these data, we made the averaged reservoir thickness and averaged porosity variation maps for this region ( Figure 6). The averaged reservoir thickness map and revised well logging profile show a thickening of sedimentary deposits from Wells C8 and C6 in the southwest to Well C2 in the northeast (Figure 6a). The averaged porosity map and the revised well logging profile show that the porosity for the sandstone of the Es 3 L Formation is lower in Wells C8, C10, and C2 at the edges and gradually increases to Wells C6 and C61 in the south-central area (Figure 6b).  Table 1; however, 1, 4, and 3 were subtracted from averaged Th/U, Th/K, and Pe data, respectively, to clarify the differences. Black lines are paleoelevations. Red arrows indicate the directions of paleocurrents.  Note: "S1-5" = Sandset 1-5. "-" = no data. a-f = data used in Figure 6b, g-l = data used in Figure 6a.  Table 3.
The well testing data from a depth interval of 3356.2 m to 4552 m show that the porosity ranges from 0.1% to 10.6% and the permeability from 0.1 to 5.535 milli-darcy (abbreviated to mD) (Figure 7; Table 4), suggesting generally poor physical properties of the layers from the Es 3 L Formation in this region. However, two positive anomalies were found from 3700-3800 m and 4100-4300 m intervals (Figure 7), suggesting improved physical properties.

Analysis of Provenance Directions
The concentrations of these heavy metals are determined by the abundance of the minerals that host them. Besides the paleocurrents, the diagenesis may also influence them. In the Chezhen depression, the sedimentary deposits experienced gentle compaction, cementation, and dissolution in the burial diagenetic evolution [68]. However, the study region was located in the center of the Chezhen depression and was shown to be a single and stable lacustrine environment [3], suggesting that the influence of diagenesis on the variations of indicators was homogeneous in different wells. The original distribution of heavy minerals was basically finished during sedimentation and syndiagenesis, and was then changed by the sustaining paleocurrent flows. Some regional studies suggested that the sedimentary layers of the Es 3 L Formation were mainly in the middle diagenetic stage A [68][69][70]. The alteration to the mother rocks was of not very significant. Analyses of lab data and rock slices suggested that the primary pores were effectively preserved, and the secondary pores affected amounted to 3-8% [69], which did not critically affect the sedimentary deposits. The paleogeotemperature was estimated to be less than 140 • C. It was at a low thermal evolution stage of the clay minerals and the tectonic zones [71]. The cementation and dissolution were not sufficient to result in an extensive migration of heavy minerals. The minerals in the deposits of the Es 3 L Formation in this region were mainly disordered [71]. Additionally, the assemblages of heavy minerals were valid in the provenance analysis even in regions with stronger diagenesis [72]. The tectonic activities were slight and infrequent after the age of the Es 3 L Formation, and the depocenters almost unchanged [1]. The sedimentary layers of the Es 3 L Formation have been kept in good condition since its formation, also suggesting a lower effect of epidiagenesis and other exogenous processes. Therefore, we suggest that the influence of diagenesis on the variations of indicators can be neglected in this study, and the heavy minerals were still effective in the provenance analysis.
Because the particles of heavy minerals are small, they are often wrapped in muds, and the concentration is enriched in different areas during the transportation of paleocurrents [58,59]. The heavy mineral analysis is also a well-established method used for the determination of palaeocurrent directions and provenance analysis [21,22]. Different regions of provenance have specific characteristics of parent rocks, which are manifested as different assemblages of heavy minerals in the sedimentary deposits transported by paleocurrents [21,22]. The gamma logging technology measures the value of the total concentration of elements from all minerals around the underground wellbores, which is a better indicator than the ones derived from individual samples [27,28]. The effect of dissolution on the results is also greatly limited. Therefore, we suggest that results obtained by GR logging and lithology-density logging can effectively represent the variation of ionic concentration after the redistribution of paleocurrents in the study area.
The results of GR logging and lithology-density logging from seven wells in this area were plotted in Figure 5. The directions of paleocurrent sources were recognized as northwest and west; northeast, northwest and west; and northeast, northwest and west, based on the variations of the Th/U, Th/K, and Pe values of Sandsets 1-5 in the Es 3 L Formation, respectively.
The results of dip logging are in good agreement with those of GR logging and lithology-density logging. Because the sedimentation and regional deformation were gentle after the formation of the Chezhen depression [1,3] and the blue pattern of dip variation was predominant in this region, the paleocurrent directions can be easily recognized by the main tendencies of the strata. Therefore, the paleocurrents from the northeast, northwest and west were identified from the results in Wells C10, C01, and C61 ( Figure 8). The variations of reservoir thickness and porosity are reliable auxiliary indexes in a region with a stable sedimentary environment and strong paleocurrents. The reservoir thickness has a high correlation with paleogeomorphology and depositional sequences. From the statistical results, the variation of reservoir thickness proves the existence of a source of sediments and paleocurrents in the further northeast ( Figure 6a). As the sediments were transported by paleocurrents, the muds and clays decreased. The porosity of the sedimentary deposits should increase from the provenance to the depocenter. The variation of porosity in this region agrees well with the inferences from well loggings, suggesting the provenance of paleocurrents from three directions (northeast, northwest and west) (Figure 6b).
Two positive abnormal zones of porosity and permeability were found at the depth intervals of 3700-3800 m and 4100-4300 m (Figure 7 and Table 4). Because the controlling action of overpressure on the reservoirs is stronger than in other reasons for the Paleogene strata in the Chezhen Depression [70], the anomalies of porosity and permeability may be caused by the overpressure of liquid reservoirs and the gas produced by organic materials. Combining these results and the provenance analysis, we suggest that high-quality liquid reservoirs should be located at the intersection of multiple paleocurrents, namely, between Wells C8 and C61 and between Wells C2 and C61, with depths of 3700-3800 m and 4100-4300 m (A and B in Figure 8).
Based on the logging data from all 40 wells (33 of them are hidden following privacy policies) in the Chezhen depression, we restored the paleogeomorphology associated with the deposition of the Es 3 L Formation in this region ( Figure 9). The paleogeomorphologic restoration map shows that the central to northeastern part of the region was the subsidence center of the Chezhen depression with lower altitude, collecting the sediments from the terrains with higher altitude in the northeast, northwest, and west. Although there is an area of middle-high paleoelevation in the southern part of this region, we consider it may be the stratigraphic overlapping of sediments deposited along the southern margin of the half-graben based on our results regarding paleocurrents (Figure 9b).

Interpretation of the Tectonic Dynamics
The macroscopical paleocurrents and sedimentary transport in basins are often controlled by the surrounding tectonic activities. In a faulted basin, the macroscopic directions of paleocurrents and provenance usually indicate the sources of tectonic stress in the region [61,73,74]. As a typical sub-basin of the Bohai Bay basin, the Chezhen depression, belonging to the Bohai-Luxi block, is adjacent to the N-S-trending Tan-Lu strike-slip fault, the Izanagi Plate, and the Pacific Plate in the east; is adjacent to the NE-SW-trending Lan-Liao strike-slip fault in the west; and it is remotely affected by the compression-stretch of the Yinshan-Yanshan fold belt and the collision of the North China block and South China block in the N-S direction ( Figure 10) [41][42][43]75]. Therefore, the multidirectional tectonic genesis makes it of great research value.
There are a series of NW-SE-ranged Cenozoic secondary basin and uplift tectonic assemblages, including the Chezhen depression, in the Bohai Bay basin, in the east of the North China block, which are referred to as the "wave basining" phenomenon [10,76]. During the Cenozoic, the mantle thermal equilibrium in the Bohai Bay area was destroyed due to the subduction of the Pacific Plate from the Eurasian Plate and the collision of the Indian Plate with the South China block. After the dissolution of the old, cold lithosphere (earlier than ca. 450 Ma) in the middle and late Mesozoic, the thermal convection in the mantle of this area was disturbed and resulted in regional uplifts, while the continental crusts above the new underplating mantle stretched and resulted in rifting depressions [77,78]. On the other hand, the plate tectonic activities and interactions were still ongoing at the same time. The Pacific Plate began subducting to Eurasia before the Jurassic [42,[79][80][81]. The velocity of subduction reached its peak at~10 cm/y at the end of the Cretaceous and then declined rapidly after the Cenozoic (ca. 50 Ma) [42,82,83]. The rapid attenuation of the subduction velocity led to the extension of the crusts on both sides of the subduction zone, resulting in a large number of collapse and pull-apart depressions [42,84,85]. The distribution of volcanic rocks decreased and migrated from the western inland to the eastern coastal area during the Paleogene, which also suggests the retracement of the Pacific Plate [86][87][88]. Despite the subduction zone of the Pacific Plate retreating on the eastern margin of the North China block, the mid-ocean ridge was still expanding [82,83]. The alternations of subductions and rollbacks of the Pacific Plate in the eastern margin of the North China block and the re-activation of the ancient mantle under the North China craton might have led to the "wave basining" phenomenon in the Bohai Bay basin. The analyses of provenance and paleocurrents indicated that the Chezhen depression received a large amount of sediments transported by paleocurrents from northwest in the age of Es 3 L , suggesting that a southeastern retracement of the Pacific Plate was happening in the early Paleogene (ca. 50.5-41.7 Ma) ( Figure 10). The observation of absolute moving velocities of the Pacific Plate relative to the lower mantle suggested its dynamic changed from nearly NW-SE to W-E [82,89], which resulted in the western provenance for the Chezhen depression.  [42,76,77]. These large-scale NNE strike-slip fault systems played an important role in the tectonic formation of the Bohai Bay basin. These strike-slip faults have very obvious piecewise evolutionary characteristics: the initial formation time of these fault groups is vague, possibly during the early Indochina period; then they were basically formed in the early Indochina period. In the late Yanshan period, most of them were left-lateral strike-slip faults; in the Cenozoic, they mostly manifested as right-lateral extensional strike-slip faults [42,76,77,89]. Among them, the Lan-Liao fault showed stable right-lateral strike-slipping during the entire Cenozoic, while the Tan-Lu fault showed left-lateral strike-slipping during the Paleocene to early Eocene and changed to right-lateral until the middle Eocene [31,77,89]. During the Cenozoic, the subduction front of the Pacific Plate reached the Bohai Bay and even further north [86,89], which intensified the strike-slip movement of the fault zones. The provenance from northeast may suggest that the Chezhen depression (Bohai-Luxi block) was subjected to the southwestward escape tectonic movement induced by the Lan-Liao and Tan-Lu strike-slip faults in the early Paleogene ( Figure 10).

Conclusions
In this study, we obtained the Th/U and Th/K ratios, Pe index, porosity, permeability, stratigraphic dips, and the thickness of reservoirs by using well logging technology on the sedimentary layers of the Es 3 L Formation in the Chezhen depression; and we reconstructed the paleogeomorphologic maps. The combined results indicate the existence of paleocurrents from the NE, W, and NW and convergence in the northern Chezhen depression at that time. Based on the well testing data, the physical properties of the layers from the Es 3 L Formation in this region were generally poor; however, two abnormal overpressure zones were found at 3700-3800 m and 4100-4300 m deep intervals, suggesting potential high-quality underground liquid reservoirs. By combining with other geological evidence, we suggest that the Pacific Plate was retreating and changing its direction from NE-SW to W-E and the Bohai-Luxi block was suffering an extrusion from NE induced by the Lan-Liao and Tan-Lu strike-slip faults in the early Paleogene.  Data Availability Statement: All data necessary to carry out the work in this paper are included in the figures, tables or are available in the cited references.