Analysis of the Main Controlling Factors of Oil and Gas Accumulation in the South China Sea Using Ya’nan Sag as an Example

Abstract

The Ya’nan sag has experienced more than 40 years of exploration, but only the Ya 13-1 large gas field has been found. In recent years, no oil or gas field has been found, and exploration work is difficult. To clarify the main controlling factors of the oil and gas reservoirs in Ya’nan Sag, the direction of oil and gas exploration is highlighted. Based on 3D seismic data combined with drilling information, this paper determines the basic geological conditions, clarifies the main controlling factors of reservoir formation, and predicts strong exploration directions. The research revealed the following: ① The source rocks of the Yacheng Formation in the Ya’nan sag are developed, and the types are II2-III, with excellent resource potential. ② In-depth analysis of the drilled wells revealed that high-quality reservoirs, oil-source faults, and caprock conditions are the main controlling factors of hydrocarbon accumulation in Ya’nan sag.

1. Geological Background

The Ya’nan sag is located west of the Qiongdongnan Basin and features an E-W trend (Figure 1). The Ya’nan sag is an important hydrocarbon generation sag in the Qiongdongnan Basin. Many years of exploration investment have made the degree of exploration in this area very high. Fault block traps controlled by the No. 1 and No. 2 fault zones have been drilled in many fields, from normal temperature and normal pressure in the northern part of the sag to high temperature and high pressure in the southern part of the sag and from structural traps to lithologic traps. However, only the Y13 gas field was found in the Y13 low uplift area in the southern area around the Y13 depression, and no large exploration area was found around the depression. As the Ya 13 gas field gradually enters the end of production, finding new gas reservoirs to replace existing production capacity has become a top priority to ensure a stable energy supply. As a potential replacement area, studying the geological characteristics, tectonic evolution, sedimentary environment, and hydrocarbon accumulation conditions of the Ya’nan sag is particularly important. Previous studies have carried out much research in the study area and achieved a series of important results [1,2,3,4,5]. In this work, the geological condition theory of oil and gas reservoir formation is adopted to analyse the Ya’nan Sag, and the main controlling factors of hydrocarbon accumulation in Ya’nan sag are analysed.

The Qiongdongnan Basin is a Cenozoic deposit tensional basin developed on the northwestern continental shelf of the South China Sea [6,7]. The west and Yinggehai Basins are bounded by the No. 1 fault, and the northeast and north are connected with the Shengu uplift and Hainan uplift, respectively. The southern part of the basin is adjacent to the Yongle uplift. Many groups of NE, NW, and nearly WE-trending basin-controlling faults have developed in the Qiongdongnan Basin [8]. The development of Paleogene basins is controlled by basement faults, and fault activity controls the formation of basin rift structure patterns and the development of sag deposits [9].

The Ya’nan sag is a hydrocarbon-rich sag in the Qiongdongnan Basin. The sag is controlled by the No. 1, 2 and 3 faults, and multiple oil subsags and secondary structures have developed. The activities of the No. 1 and No. 3 faults controlled the development and distribution of hydrocarbon source rocks in the Ya’nan sag. The development and evolution of the control No. 3 fault in the northern Ya’nan sag is controlled by the control No. 3 fault, resulting in the structural pattern of the Ya’nan sag as a whole. During the Paleogene period, No. 1 and No. 3 began to form due to the extrusion effect of the Indochina block, the left-lateral strike-slip motion of the Red River fault, the subduction of the ancient South China Sea, and the expansion of the new South China Sea [10]. The E-W-trending No. 3 fault continued to be active in the late Oligocene, and a series of faults in the same direction were derived inside the sag. The formation and development of these faults control the distribution of oil and gas reservoirs in the sag.

The Qiongdongnan Basin is a Cenozoic rift basin developed from the Paleogene basement [9]. Depositions are mainly composed of Paleogene, Neogene, and Quaternary deposits, and the Neogene depression and Paleogene fault depression are divided into two sedimentary structural layers with T60 as the boundary (Figure 2). It presents a double-layer structure of downfault and updepression [11]. The upper tectonic layer comprises the Sanya Group, Meishan Group, Huangliu Group, and Quaternary strata, and the lower tectonic layer comprises the Lingshui Formation, Yacheng Formation, and Eocene strata [12]. The coal-bearing strata are mainly found in the Paleogene Yacheng Formation and Lingshui Formation, which are the main source rocks in the Ya’nan sag [13,14]. The Ya’nan sag is located west of the Qiongdongnan Basin. The area west of the sag and Yinggehai Basin is bounded by the No. 1 fault. The northern boundary is separated by the No. 3 fault and Yacheng uplift. It is connected to the Ya’nan low uplift in the south and to the junction of the Yacheng uplift and Ya’nan low uplift in the east. It is generally distributed in the northeast to nearly east–west direction (Figure 3), wide in the west and narrow in the east, forming an inverted triangle [15].

2. Oil and Gas Geological Analysis in the Study Area

2.1. Structural Style and Evolution

The structural style is the sum of the structures produced by the three-dimensional space under the same regional stress field or the same period of structural deformation. The factors influencing structural deformation include faults, folds, and their arrangement and combination. For a basin with a cyclic evolution history, the development characteristics of the structural style are superimposed [16].

The structural style of the Ya’nan sag is mainly semi graben. The structural style of the Ya’nan sag has experienced multistage fault superposition activity and local inversion, resulting in more complex structural combinations. The structural geometry of the Ya’nan sag is a dustpan structure with a north fault and south overlap, and the sedimentary thickness gradually decreases from north to south. The characteristics of the fault system are as follows: basement subsidence is mainly controlled by the No. 3 depression-controlling fault, which controls the development of the steep slope zone on the north side of the depression; NW-trending and nearly EW-trending secondary faults are developed inside the sag; the development of secondary faults forms a fault block structure (Figure 4); and the sag is divided into multiple secondary sags. During the tectonic evolution of Ya’nan sag, tensile stress was dominant. Therefore, the Ya’nan sag is dominated by extensional tectonic stress. The structural styles of the Ya’nan sag include semi graben type, positive fault step, negative flower shape, Y-shaped, and other combination types.

An analysis of the tectonic evolution of the Ya’nan sag (Figure 5) reveals that the extension activity of the Ya’nan sag during the tectonic period first increased but then weakened. During the substantial extension during the Eocene–Oligocene period, the first expansion of the South China Sea triggered terrestrial extension, and the regional stress was dominated by NW-SE tensile stress. The extension caused vertical differential sliding of the fault and formed a half-graben structure. The secondary faults are arranged in an echelon pattern, controlling the formation of subsags and uplifts. During the weak extension-strike-slip transition in the Miocene period, the second expansion of the South China Sea led to the opening of the southwestern subbasin, and the regional stress changed to N-SW extension and was superimposed on the right-lateral strike-slip component. The activity of the sag weakened, the local faults adjusted to form a negative flower structure, and a regional unconformity surface developed. From the late Miocene to the present, the expansion of the South China Sea stopped. Affected by plate movement, the stress turned into weak compressive stress, and the compressive anticline structure was formed by compression, which provided a place for oil and gas accumulation.

2.2. Characteristics of the Stratigraphic Framework

To analyse the stratigraphic development characteristics and stratigraphic framework styles in the study area systematically, this study selected two seismic profiles that passed through key structural parts for the construction and analysis of the stratigraphic framework, including one NW-trending profile and one SW-trending profile.

The section shown in Figure 6 is located west of the HuanYa’nan sag. The section starts from the Yabei sag, passes through the Yacheng bulge, the Ya’nan sag, and the Ya’nan low bulge, and ends in the Ledong sag. The whole fault dips south, and a few faults dip north. The strata of the Tuoyacheng Formation are distributed mainly in Yabei sag, Ya’nan sag, Ya’nan low uplift and Ledong sag. The Yacheng uplift lacks the Yacheng Formation strata, and the Ledong sag deposits the thick Yacheng Formation strata. The strata of the Lingshui Formation are distributed throughout the whole area. The No. 3 fault downthrown plate deposits a very thick Lingshui Formation, which is denser than the Yacheng Formation, and the No. 2 fault downthrown plate Lingshui Formation is thinner than the Yacheng Formation. Controlled by the No. 2 fault and No. 3 fault, the strata are distributed in a stepped shape from north to south, and some layers in the Ya’nan low uplift are eroded. Ladder-like faults developed in the southern low uplift area of the cliff after the detachment process. The direction of the fault is consistent with the direction of the No. 2 fault, and the thickness of the Lingshui Formation gradually decreases.

Figure 7 shows a NE-trending section that is located in the northern part of the HuanYa’nan sag, Guoya 13-1 low uplift, Ya’nan sag, and Yacheng uplift. The strata of the Luolingshui Formation are stable throughout the whole area, and the strata in the sedimentary centre of the Ya’nan sag are the thickest and gradually thin to both sides. The strata of the Tuoyacheng Formation are distributed mainly in the Ya13-1 low uplift and Ya’nan sag. The sedimentary characteristics of the strata are similar to those of the Lingshui Formation, and the thickness gradually decreases from the centre to both sides. The profile reveals that the strata of the Lingshui Formation and Yacheng Formation are stable in the study area. Overall, the sedimentary thickness of the Lingshui Formation is relatively large, and the thickness of the Yacheng Formation is relatively tiny. The thicknesses of the two strata groups decrease from the middle to both sides.

2.3. Fault Development Characteristics

The southern sag is controlled by the No. 1 fault in the west, the No. 3 fault in the north, and the No. 2 fault in the south. There are many secondary faults in the sag, and the fault strike is similar to that of the NW-trending No. 1 fault and the nearly E-W-trending No. 3 fault. The control No. 1 fault in the western part of the depression has a complex structural style, which is a bilateral basement fault-step graben (Figure 8). It is controlled by the No. 3 fault of the control sag, resulting in the half-graben fault depression structure of the northern fault and the southern supertectonic pattern in the Ya’nan sag (Figure 9).

The No. 1 fault is the boundary fault between the Yinggehai Basin and the Qiongdongnan Basin. It is considered to be the extension of the Red River fault in the South China Sea [17,18] and has a strike-slip nature. The fault formed in the early Oligocene. The fault trends NW–SE and tilts westward as a whole. The No. 1 fault experienced a large drop in the Paleogene period, and the southward detachment on both sides of the east and west of the fault was uneven. Affected by the formation of the No. 1 fault, a series of near NW-trending small faults and derived faults are generated. The No. 1 fault is the main fault that formed in the basement. Its branch faults and adjustment faults complicated the structure of the sag, resulting in the formation of many small dustpan-like fault depressions in the west of the sag. The NW-trending faults provide migration channels for the source to enter the Ya’nan sag and control sediment transport on the west side of the sag.

Fault 2 is located in the middle of the Qiongdongnan Basin and runs through the east and west of the basin. The fault is divided into western, interrupted, and eastern sections. The western section of the No. 2 fault is bounded by the Ledong sag and Ya’nan low uplifts, and the Ya’nan low uplift is located in the southern section of the Ya’nan sag. The western section of the fault strikes nearly E–W, and its western end is connected with the No. 1 fault and intersects with the No. 3 fault to the east. The No. 2 fault in the study area has a section shape from steep to slow, with a large plane extension scale and downward convergence, forming multiple fault terrace structures. The small faults associated with the main section of the No. 2 fault form multiple small dustpan-shaped fault depressions (Figure 10).

The No. 3 fault is located in the northern part of the Ya’nan sag. The fault formed during the Oligocene. The fault strike is nearly E–W, and the strata are inclined southward. The No. 3 fault separates the Ya’nan sag from the Yacheng uplift, and the fault controls the deposition and evolution of the sag [19]. The overall structure of the southern sag is a half-graben-like half-graben fault depression structure with a north-faulting and south-superstructure pattern. The No. 3 fault on the north side of the sag is steep in the upper part and slow in the lower part, and the strata on the south side overlap in the southern low uplift. The sag has a half-graben shape, and the strata are thick in the north and thin in the south. The Paleogene Yacheng Formation and Lingshui Formation were deposited thickly.

Through the quantitative analysis of the fracture growth index and other parameters in the study area [20], the characteristics of fracture evolution in the study area can be clarified and the time of tectonic evolution in the study area can be further inferred. When the growth index is positive, the thickness of the upper strata is greater than that of the lower strata and the basin structure is in a stage of extensional evolution. In contrast, when the growth index is negative, the thickness of the hanging wall is smaller than that of the footwall and the basin structure is in the stage of extrusion deformation evolution.

In this study, three concave faults in the study area were selected for analysis. By comparing the growth indices of three faults at different positions and different periods, the evolution characteristics of concave faults were further analysed.

The calculation point of the No. 1 fault is selected from north to south, and the calculation points of the No. 2 fault and No. 3 fault are selected from west to east (Figure 11). The growth index was used to analyse concave fractures. The results are as follows:

The quantitative analysis of the growth index of the No. 1 fault from the Neogene Meishan Formation to the Paleogene Yacheng Formation (Figure 12) reveals that the activities of the northern and southern sections of the No. 1 fault differ. The main activity period of the north section of the No. 1 fault was during the deposition of the Lingshui Formation, and the main activity period of the southern section of the fault was in the Yacheng Formation. During the period of No. 1 fault activity, the basin was against the background of tectonic extension, and a series of extensional faults developed that were more active in the Paleogene Yacheng Formation and Lingshui Formation. Tectonic inversion did not occur in the Yacheng Formation in the southern section of the fault, and the growth index was negative. The reason for this phenomenon may be that extrusion and inversion occurred at the top interface T70 of the Yacheng Formation, and the hanging wall of the formation was thrust along the fault, resulting in more significant erosion of the hanging wall of the Yacheng Formation than the footwall.

The quantitative analysis of the growth index of the No. 3 fault from the Neogene Meishan Formation to the Paleogene Yacheng Formation reveals that (Figure 13) the activities of the eastern and western sections of the No. 3 fault are different. The main activity period of the west section of the No. 3 fault was during the deposition of the Lingshui Formation, and the main activity period of the fault was in the Yacheng Formation. During the period of No. 3 fault activity, the basin was against the background of tectonic extension, and a series of extensional faults developed, which were more active in the Paleogene Yacheng Formation and Lingshui Formation.

The quantitative analysis of the growth index of the No. 2 fault from the Neogene Meishan Formation to the Paleogene Yacheng Formation revealed that (Figure 14) the difference in activity between the eastern and western sections of the No. 2 fault is apparent. The main activity period of the western section of the No. 2 fault was the sedimentary period of the Yacheng Formation, and the main activity period of the eastern section of the fault was the Lingshui Formation. In the two sections of the hanging wall of the No. 2 fault, the thick Lingshui Formation and Yacheng Formation were deposited. During the period of No. 2 fault activity, the basin was against the background of tectonic extension, and a series of extensional faults developed, which were more active in the Paleogene Yacheng Formation and Lingshui Formation.

2.4. Source Rock Characteristics

The Ya’nan sag experienced rifting and depression stages, and the coal-bearing source rocks of the Yacheng Formation developed during the rifting stage. During the Oligocene, the activity of the depression-controlling faults was strong and the amplitude of the differential rise and fall was large. An obvious transgression occurred in the Yacheng period [21]. Affected by sea level changes, coarse-grained sediments migrated to the edge of the depression and fine-grained sediments were widely developed. Under these conditions, the organic matter is well preserved and the organic matter abundance of the source rock is further improved.

Combined with the drilling information and geochemical analysis, the gas generation intensity of Ya’nan sag is high and the natural gas has high maturity [22]. The source rocks of the Ya’nan sag are coal, carbonaceous mudstone, and the dark mudstone of the Yacheng Formation, which developed mainly in fan deltas and coastal plains. Combined with the three-terminal element diagram of the organic macerals of the Yacheng Formation in the Ya’nan Sag (Figure 15), the organic matter of the Yacheng Formation is type II2~III, mainly type III, with a small amount of type II1 distribution. The source rock is close to the source area and has a high plant source. There are typical coal-measure source rocks in Ya’nan sag [23]. The microscopic organic component of the coal seam is mainly vitrinite, and the content of inertinite is very low, which indicates that the coal-bearing source rocks have a high hydrocarbon generation capacity.

The basis for evaluating the hydrocarbon generation capacity of source rocks is the abundance of organic matter in source rocks, which is affected mainly by the sedimentary environment, maturity, and other factors. The coal-bearing strata are characterized by high organic matter contents and poor organic matter types. Therefore, Chen Jianping and Li Xianqing proposed corresponding evaluation criteria for organic matter abundance (

Table 1. Evaluation criteria for the hydrocarbon generation potential of coal-measure source rocks.

Type of Soure Rock	Good	Better	Worse	Bad	Sphere of Application
TOC%	>3.0	1.5–3.0	0.5–1.5	<0.5	Dark mudstone
A%	>0.1	0.05–0.1	0.01–0.05	<0.01
HC (10−6)	>400	150–400	50–150	<50
S1 + S2 (mg/g)	>6.0	2.0–6.0	0.4–2.0	<0.4
A%	>1.0	0.6–1.0	0.2–0.6	<0.2	Coal and carbonaceous mudstone
HC (10−6)	>5000	2000–5000	500–2000	<500
S1 + S2 (mg/g)	>150	80–150	20–80	<20

) and divided the source rocks of the Yacheng Formation into coal, carbonaceous mudstone, and dark mudstone for analysis.

Based on previous experimental data from rock pyrolysis tests [15] and the results of rock pyrolysis research (

Table 2. Statistical table of organic matter abundance of coal-measure source rocks in the Yacheng Formation of Ya’nan sag.

Well	TOC%	S1 + S2 (mg/g)	HI	A%	HC (ppm)	Type
Y-1-3	29.990 (3)	53.203 (3)	160.626 (3)	/	/	Coal and carbonaceous mudstone
	1.388 (6)	2.414 (6)	86.058 (6)	/	/	mudstone
Y-1-6	21.883 (7)	36.931 (7)	103.757 (7)	/	/	Coal and carbonaceous mudstone
	1.482 (17)	2.678 (17)	118.061 (17)	/	/	mudstone
Y-2-1	51.235 (2)	/	/	1.400 (1)	5600.0 (1)	Coal and carbonaceous mudstone
	/	/	/	0.163 (1)	667.070 (1)	mudstone
Y-1-2	33.007 (6)	62.673 (6)	212.821 (6)	/	/	Coal and carbonaceous mudstone
	3.851 (10)	5.904 (10)	109.254 (10)	/	/	mudstone
Y-1-1	76.125 (2)	120.6 (1)	153.066 (1)	1.745 (2)	9386.5 (2)	Coal and carbonaceous mudstone
	1.153 (6)	1.295 (6)	82.056 (6)	0.079 (4)	506.885 (4)	mudstone

), combined with the analysis of organic matter abundance evaluation criteria, the organic matter abundance of source rocks in the study area is high. By observing and analysing the frequency distribution, the average TOC content of coal and carbonaceous mudstone is between 22% and 76%. In contrast, the TOC content of dark mudstone is between 0.7% and 3.8% (Figure 16). To evaluate the hydrocarbon generation potential of source rocks, parameters such as the hydrocarbon generation potential, chloroform bitumen A content and total hydrocarbon content should be comprehensively considered. The average S1 + S2 content of coal and carbonaceous mudstone in the study area is between 36 and 120 (mg/g), and the average S1 + S2 content of dark mudstone is between 0.7 and 2.7 (mg/g). The chloroform bitumen A of coal and carbonaceous mudstone is between 1.4% and 1.7%, and the chloroform bitumen A of dark mudstone is between 0.07% and 0.2%. The total hydrocarbon content of coal and carbonaceous mudstone is as high as 5600–9386 ppm, and the total hydrocarbon content of dark mudstone is as high as 506–667 ppm. The Yacheng Formation in the Ya’nan sag has better source rocks and more significant hydrocarbon generation potential.

A map of the organic matter distribution in Ya’nan sag was obtained via comprehensive analysis (Figure 17). Since the beginning of stratigraphic deposition, organic matter has been stored in sediments as macerals, so the content of macerals affects the abundance of organic matter. It is generally believed that the hydrogen-rich component is a favourable hydrocarbon-generating component. The H/C ratio of the source rocks in the Ya’nan sag is between 0.4 and 1.2, and the O/C ratio is between 0.06 and 0.2. The hydrogen index is high, and the maturity is high.

2.5. Reservoir Characteristics

Sandstone reservoirs are mainly developed in the southern sag of the cliff. The sandstone content is generally greater than the mudstone content, and the sandstone content can reach up to 78% [24].

The Eocene, Yacheng Formation, and Lingshui Formation are developed in the Paleogene of Ya’nan sag.

The Eocene belongs to non-marine deposits and is the product of the early fault depression. The Yacheng Formation belongs to the late period of the fault depression. The early period is the transitional facies of the sea and land, and the middle and late periods are the coastal facies deposition, which is an important source rock section of the basin with a large area of swamp environment [25]. The Lingshui Formation belongs to the deposition of the late fault depression and is divided into three sections; the third member of the Qinling Formation is mainly composed of conglomerate and sandstone. It is a fan delta sandstone reservoir with a wide distribution area that extends to the Ya’nan low uplift and basically covers the whole Ya’nan sag.

The second member of Ling is dominated by mudstone, with thin sandstone; the first member of the Lingshan Formation is gravel, medium-coarse sandstone, and mudstone with unequal thickness interbedded with outputs (the unconformity surface characteristics at the top are obvious). The Neogene formations mainly include the Sanya Group, Meishan Group, Huangliu Group, and Yinggehai Group. On the whole, it is a shallow-sea, half-deep-sea sedimentary system, and the reservoir has not yet developed. The main reservoir bodies are turbidite fans, channels, and canyon deposits (Figure 18).

Therefore, large delta deposits have developed in the third member of the Lingshui Formation in the Ya’nan sag, and sand bodies are widely distributed.

2.6. Caprock Characteristics

During the Neogene period, the Ya’nan sag was dominated by mudstone. The mudstone of the Meishan Formation contains calcium and high pressure and is a high-quality caprock. The caprock of the Meishan Formation experiences physical sealing, hydrocarbon concentration sealing, and under compaction sealing. The mudstone of the Meishan Formation is stably and continuously distributed on the plane, and the exploration wells around the sag reveal this set of mudstone. The thickness of the Meishan Formation mudstone is generally 110–310 m, and the thickest part is 500 m thick, which is a set of high-quality regional cap rock. The main gas layer of the YC13-1 gas field is located under this cap rock.

2.7. Trap and Preservation Conditions

The faults in the study area have developed and the fault combination style and superposition relationship of the Ya’nan sag are complex. Therefore, the traps found in the study area are controlled by faults. Structural traps, fault block traps, stratigraphi–clithologic traps, etc., have been found in the study area. The types of traps are diverse, the trap control area is large, and the potential for natural gas exploration is considerable.

The large set of mudstones in the Meishan Formation in the research area provides sealing conditions and is good for the Lingshui Formation and Yacheng Formation gas reservoirs. The fault and mudstone layers form a good fault–capping relationship, which is avail to the preservation of oil and gas reservoirs.

2.8. Migration Conditions

A comprehensive analysis revealed that oil and gas migration in the Ya’nan sag mainly depends on fractures and sand bodies [26]. The fault is the most important transport system in the Ya’nan sag. Most of the faults in the southern sag of the cliff are developed below T60, and concave-controlled faults at the edge extend above T60. The vertical transport of faults is reflected mainly in the Paleogene structural layer. Most of the faults ceased to develop before Neogene activity, resulting in these faults cutting the Paleogene sandstone transport system. There are large fan delta or delta sand bodies in the southern sag of the cliff, and the sand bodies are widely distributed. The basement fault connects the source rock and the sand body, the oil and gas migrate along the fault to the sand body and primary migration occurs. Under the influence of fault activity, the enriched oil and gas in the sand body migrated upward. In addition, sand bodies in direct contact with source rocks can also be used as important channels for oil and gas migration.

3. Analysis of Drilled Wells

The analysis of drilled wells is an important part of oil and gas exploration, and the results are directly related to the exploration direction of an area. The Ya’nan sag is an important hydrocarbon generation sag. After several years of exploration, more than 10 wells have been drilled. Combined with stratigraphic interpretation data and logging data, the analysis of three failed wells and one gas well in the Ya’nan area helps us better understand the accumulation model of Ya’nan sag.

3.1. Drilling Analysis of Failed Wells Y-1-9 and Y-2-1 and Gas-Producing Well Y-1-1

The Y-1-9 well, Y-2-1 well, and Y-1-1 well are located in the Ya-13 low uplift. The No. 13 uplift is located at the western end of the Ya’nan sag, and the Ge No. 1 fault is adjacent to the Yinggehai Basin. The two sags have a high organic matter abundance and are in a stage of hydrocarbon generation and expulsion, which can provide a dual-source hydrocarbon supply for the Ya 13 low uplift, which is conducive to oil and gas filling [2]. The Ya 13 low uplift is an inherited ancient uplift in which the residual hills and gullies are intertwined. The Yacheng Formation overlaps the residual hills and fills the gullies. The deposition is thin, the sediment is coarse, the reservoir conditions are good, and the local structure is developed at multiple levels. Influenced by the No. 1 fault and No. 3 fault, a series of NW-trending faults and nearly E-W-trending faults developed in the Ya 13 low uplift, and the faults were staggered, which is conducive to the formation of fault block traps. Coarse sand and fractures form a good transport system that is conducive to the migration of oil and gas.

The main gas reservoir in the southern sag is the sandstone gas reservoir of the third section of the Lingling Formation. The Y-1-9 well is drilled at the high position of the anticline structure, and the sandstone formation is updippinched out at the low position of the Ya 13-1 anticline. As a result, the Y-1-9 well lacks gas-bearing sandstone reservoirs, resulting in exploration failure (Figure 19).

The Y-2-1 well was drilled on the downthrown side of the fault. The Y-2-1 well was separated from the Ya’nan hydrocarbon generation sag by a buried hill structure. Therefore, the gas source of Ya’nan sag could not have migrated to the Y-2-1 well. The reason for the failure of this well was migration (Figure 20).

Well Y-1-1 is an important gas well in the Ya13 low uplift. Located in the high area of oil and gas aggregation in the Ya’nan sag, the interlaced faults are favourable to forming of fault block traps. Most faults developed only during the Paleogene, and oil and gas relied on faults to accumulate in the third member of the Lingshui Formation and did not diffuse upward along the faults. Drilling confirmed that the Meishan Formation is dominated by mudstone, which is the cap rock of the Lingshui Formation and other reservoir areas. The burial depth of well Y-1-1 is relatively shallow, and the lithology of the third member of the Lingshan Formation is mainly medium-coarse sand. The high porosity and permeability make the third member of the Lingshan Formation a high-quality reservoir. The reservoir is directly connected with the cap rock of the Meishan Formation, which is conducive to the protection of traps (Figure 21).

3.2. Drilling Analysis of the Failed Well Y-1-1

Well Y-2-1 is located in the Ya’nan low uplift. In terms of sedimentation, Ya’nan sag has obvious coarse filling characteristics in the north and fine filling characteristics in the south, and the sandstone development conditions in southern Ya’nan sag are poor. Therefore, the reservoir conditions of the Ya21-1 structure are poor. The main transport layer of Y-2-1 is the sandstone of the third section of the Lingshui Formation, the grain size of the sandstone is fine, and the transport capacity is poor [27]. The pressure in the Ya’nan sag gradually increased from north to south, and it was difficult for oil and gas to accumulate in the Ya 21-1 structural belt. The failure of well Y-1-1 is due to poor reservoir development, the poor physical properties of the carrier layer, strong overpressure, and problems in oil and gas migration (Figure 22).

4. Analysis of Main Controlling Factors of Accumulation

The controlling factors of hydrocarbon accumulation include high-quality source rocks, good reservoir conditions, migration channels, tectonic activities, and caprock conditions. Combined with drilling information, the main factors controlling hydrocarbon accumulation in Ya’nan sag are analysed. The analysis is as follows:

A favourable reservoir, the Y-1-1 well, is located in the Ya13-1 gas field and has the characteristics of a good reservoir facies belt, thick sandstone, and good physical properties. The Y-1-9 well adjacent to the gas field is similar to the Y-1-1 structure, but the Y-1-9 reservoir is a delta front deposit. The sandstone has a small particle size and poor physical properties, the thickness of the sand body along the source direction gradually decreases, and oil and gas accumulation lacks favourable reservoirs.

Second, good oil and gas migration channels exist. The source–reservoir transport communication of the sag mainly depends on faults and sand bodies with good connectivity. Well Y-2-1 is blocked by buried hills to prevent oil and gas migration. The grain size of the transport layer in well Y-1-2 is fine, the transport capacity is poor, and there is overpressure, which affects oil and gas migration.

Three cap rock conditions, namely, the Ya’nan sag hydrocarbon source, reservoir and migration conditions, are good, and a comprehensive analysis of the main controlling factors of oil and gas accumulation reveals cap rock conditions. According to the study of drilled gas wells in the southern sag, the traps directly covered by the cap rock in the reservoir are effective and the traps without direct cap rock are ineffective (Figure 23). The Ya 13-1 gas field is characterized by the sealing of the Meishan Formation, and the lithology is mainly calcareous mudstone, which has three possible physical properties: sealing, hydrocarbon concentration sealing, and under compacted fluid sealing.

5. Conclusions

(1)

The hydrocarbon generation conditions in the southern sag are good, the source rocks of swamp facies are developed, the organic matter content is high (the organic matter content is type II~III), and the amount of hydrocarbon generation is high. Previous analyses of the factors controlling hydrocarbon accumulation in the sag are unclear, resulting in poor exploration effects.

(2)

Through in-depth systematic analysis of drilled wells, high-quality reservoirs connecting oil source faults and caprock conditions are considered the main controlling factors of hydrocarbon accumulation in Ya’nan sag.