Based on the studies of sedimentary facies, oil-source correlation, formation pressure structure, homogenization temperature of fluid inclusions, etc., the oil and gas accumulation model and main controlling factors in lower Yanchang Formation in Wuqi-Dibian area have been discussed. It is believed that two sets of source rocks, C7 and C9, are developed in study area, and hydrocarbon produced from layers of C8 and C9 is mainly from C7 source rock, followed by C9, according to oil-source correlation; hydrocarbon-generating pressurization of C7 source rock is the main driving force for the downward migration of oil. The high value area formed by the low value of overpressure difference between C7 and C8 is the main hydrocarbon accumulation area; deltaic front subaqueous distributary channel and mouth bar in lower Yanchang formation are the main accumulation spaces due to their good porosity and permeability; besides, C8 reservoir shows the characteristics of “episodic filling and continuous accumulation,” and they both are the undersource reservoir-forming combination; it is believed that the distribution of oil reservoir in Triassic series is controlled by the factors of “near source, low pressure, superior facies.”
Ordos Basin is a cratonic hydrocarbon-bearing basin with polycyclic sedimentary, and it is also the second largest sedimentary basin in China due to its prolific hydrocarbon resources [
In recent years, commercial oil flow has been obtained through the lower reservoir group combination prospecting within the structural belt of Shaanbei slope and Tianhuan depression in Ordos Basin, such as reservoirs of Xifeng, Jiyuan, Wuqi, and Dingbian, among which the main production layer is the Chang8 (C8) reservoir group. The discovery of lower combination shows the prolific hydrocarbon resources in the layer and has been paid much attention. However, the previous research was mainly focused on the sedimentary facies and reservoir characteristics of lower Yanchang formation [
Wuding area is mainly located in the middle zone of western part of Yishaan slope in Ordos Basin and across over the eastern edge of Tianhuan depression (Figure
Structural location of study area in the Ordos Basin.
Study area locates in the middle zone of western edge of the basin, and the lower Yanchang formation belongs to fluvial-lacustrine-deltaic deposit system where mainly develops shallow water delta and deltaic front subaqueous distributary channel deposits. The most favorable sedimentary facies belts are deltaic front subaqueous distributary channel and channel mouth bar sands. Sand bodies from delta front are relatively high in both the structural and compositional maturity due to the fluvial and lacustrine transformation effect. Besides, in addition to corrosion effect, these sand bodies were high in original porosity before sedimentary happens, and thus, secondary dissolution pore developed forming accumulation spaces with good porosity and permeability.
Because of the irreversibility of the sediment compaction effect, the fluid pressure calculated by the equilibrium depth method from the compaction curve reflects the formation pressure distribution under the state of maximum buried depth in this area. Due to the influence of regional-sedimentary history, most areas of the Ordos Basin had the maximum burial depth at the end of the Early Cretaceous, so the calculated excess pressure more reflected the situation of this period. In this study, the acoustic time difference values of 16 representative Wells in each area were selected, and the normal compaction trend line was drawn on the curve of relation between acoustic time difference and buried depth. In this paper, the acoustic time difference values of 160 Wells were read, of which 9 Wells had no abnormal pressure. Combined with the parameters of C value in each zone, the acoustic time difference values of 151 Wells were calculated by the recovery of equilibrium depth, formation pressure, hydrostatic pressure, and excess pressure. According to the maximum formation pressure of a single well, the overpressure plane and profile of Chang 7 are compiled.
Mesozoic source rock series in Ordos Basin are mainly Triassic mudstone, and shale in Yanchang formation distributed within the area of
Lithological columnar section of study area.
According to the maturity evaluation index of organic matter and tested data (Table
Organic geochemical parameters of C7 members in study area.
Layer | “A” (%) | Max peak temperature | Total organic carbon | Potential hydrocarbon generation amount | Saturated hydrocarbons (%) | Aromatic (%) | Conversion rate of asphalt “A” (%) | Ro (%) | OEP | |||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Rage | Average | Rage | Average | Rage | Average | |||||||
Chang7 | 0.1 ~ 1.0 | 450~470 | 2.45~5.28 | 2.51~7.10 | 14.2 ~ 65.5 | 19.57~21.8 | 3.60~41.59 | 12.86 | 0.85~1.18 | 1.07 | 0.99~1.09 | 1.02 |
Hydrocarbon (saturated and aromatic) amount of asphalt “A” of the source rocks in C7 is between 45% and 70%, while the ratio of saturated and aromatic hydrocarbon is between 0.72 and 3.7 (Table
The following conclusions are made after the C7 sample test of hydrocarbon source rock: (1) rearrangement hopanoid C30 < hopanoid C30; (2) Tm<Ts (Ts represents 18
Characteristics of terpene hydrocarbon and sterane distribution of C7 source rocks of Dingtan3155 well in study area: (a) terpane; (b) sterane.
It is seen from the comparison graph of the symbol compounds, terpane and sterane biomarkers, that hopanoid C30 is predominant in C8 crude oil, and the amount of Ts is relatively high while that of gammacerane is relatively low. C27 regular sterane is high in amount, and C28 and C29 show the distribution of asymmetric V-shaped [
Comparison graph of biomarker terpene and sterane hydrocarbon of crude oil of C8 of C46 well: (a) terpane; (b) sterane.
Homogenization temperature test of fluid inclusions has been conducted to analyze the hydrocarbon filling time in study area showing that the homogenization temperature continuously distributes in a wide area and between 70 and 170°C; this means that during the filling process, none of big tectonic events have happened, showing the characteristics of continuous hydrocarbon filling. An obvious temperature peak zone is shown between 80~100°C and 120~130°C (Figure
Histogram showing homogenization temperatures of fluid inclusions of C8 in study area.
According to the analysis of rock samples, the lithology of the C8 reservoir in Wuqi-Dingbian Oilfield is mainly light to dark gray, fine to very fine-grained lithic arkose, and less arkose and feldspathic litharenite. The average content of quartz, feldspar, and debris is 31.05%, 34.18%, and 17.54%, respectively. The main composition of debris is metamorphic debris, followed by igneous debris, and the content of unstable and plastic debris is high. The fillings in the reservoir sandstone are high, with an average content of 15.41%. The fillings are mainly iron calcite (4.55%), followed by chlorite (3.65%) and kaolinite (2.78%) (Figure
Cement and pore characteristics. (a) Kaolinite cements, D 1184 well, 20(-), C8. (b) Ferric calcite cement, 10(+), D 1729 well. (c) Turbidite cementation, 10(+), C 3604 well, C8. (d) Ingranular and intergranular, (-) 10, 81, D 4225 well. (e) Intergranular solution hole, 10(-), 4267 well, C8. (f) Intergranular solution pore of zeolite, 3604 well, C8.
According to core data from 145 wells, the porosity of the C8 reservoir is 6%-17.12%, mainly 8.0-12.0%, average 7.9%, and median 9.1%; the permeability is 0.1 ~
Formation pressure structure is the vertical development characteristics of overpressure system and the transition relationship between overpressure system and overlying normal pressure system. Formation pressure structure is closely related to oil-gas accumulation and preservation [
Using the abovementioned method, the paleopressure and residual pressure at the key accumulation period (late Early Cretaceous) were calculated. Figure
Residual pressure profiles of well Ding1188-well D ingtan4986 of Yanchang Group in. Wuqi-Dingbian.
Through the study of source rock distribution, reservoir distribution, abnormal pressure, and reservoir forming mechanism, it is concluded that the controlling factors for oil-gas accumulation are “near source, superior facies, low pressure.”
Source rock is the material basic for reservoir forming. It is believed that the dark mudstone deposited during C7 period of the Yanchang group in Ordos Basin is the main hydrocarbon source rock with the characteristics of high thickness, wide distribution, high organic matter amount, good in organic matter types, and high maturity [
Controlled by hydrocarbon source rock, sandstone deposited during C8 period, which is close to C7 dark mudstone, captures relatively high amount of oil and gas and thus more reservoir forms. It is seen from the number of wells in C8-C10 in the Yanchang group obtaining commercial oil flow and the distance they are away from C7 bottom (Table
Relationship between the number of commercial oil wells of C8-C10 and the distance of C7.
Distance to C7 bottom (m) | 0 ~ 15 | 15~30 | 30~45 | 45~60 | 60~75 | 75~90 | 90~105 | 105~120 | Total |
---|---|---|---|---|---|---|---|---|---|
The number of commercial oil wells | 19 | 12 | 12 | 8 | 5 | 6 | 3 | 2 | 67 |
Reservoir development is closely related to the sedimentary system and sedimentary facies. Superior sedimentary and diagenism facies are advantages of forming thick reservoir with good physical properties which favors oil accumulation. Research shows that the high thickness and strong anticompaction capacity of sand body favor the preservation of intergranular pores resulting in high surface porosity, good connectivity among pore throat, and good physical properties, and these are the main areas for oil-gas accumulation. In study area, deltaic front subaqueous distributary channel and channel mouth bar are where deltaic sand body develops, especially the deltaic front subaqueous distributary channel where the sand granularity is relatively coarse, and the sand has been through the fluvial and lacustrine transformation effect leading to good sorting. Thus, pores are well developed, and permeability is good leading to good reservoir physical properties.
High overpressure difference is prevalent in C7 and C8 formation in study area, and it is the main driving force for oil migrating downward. Under the control of overpressure, oil migrates from high pressure zone to low pressure zone. Thus, low pressure zone with a high pressure background is the main space for oil and gas accumulation.
During middle and late early cretaceous, research has been through regional great subsidence, and the new deposition happened reaching its maximum until early late cretaceous [
Through the study of abnormal pressure, it is proved that the reservoir in the Yanchang group in middle zone of the southern part of Ordos Basin is “upper source and lower reservoir formation” [
Hydrocarbon has migrated both vertically and horizontally. Under the abnormal pressure, oil-gas produced from C7 source rock partially moved upward forming reservoir of which C6 is its main accumulation space, and the other part moved downward forming C8 reservoir and even reached C9 where overpressure is high. Further research shows that there are four reservoir forming types: (1) the reservoir comes into direct contact with source rock, (2) oil and gas are transported and accumulated by superimposed permeate sand body, (3) oil and gas are transported and accumulated through fluid pressure cracks, and (4) oil and gas are transported and accumulated through superimposed permeate sand body and fluid pressure cracks.
Oil source correlation shows that oil and gas in C8 and C9 formation in Wuding area mainly come from hydrocarbon source rocks in C7 followed by that in C9 Pressure increase due to hydrocarbon generation in C7 formation in the Yanchang group in study area is the main driving force for oil downward migration. The relatively low pressure zone developed during C7 and C8 period of early cretaceous is the favorable direction of oil and gas migration. The high value area of C7 and C8 with a background of low overpressure difference is the main space for oil and gas accumulation in lower combination of the Yanchang group C8 and C9 reservoirs in the Yanchang group in study area show the characteristics of “episodic filling and continuous accumulation,” and they both belong to the same reservoir-forming combination, that is, undersource reservoir-forming combination. Four main reservoir forming types are developed in lower combination: the reservoir comes into direct contact with source rock, oil and gas are transported and accumulated by superimposed permeate sand body, oil and gas are transported and accumulated through fluid pressure cracks, and oil and gas are transported and accumulated through superimposed permeate sand body and fluid pressure cracks Through the research of sedimentary, reservoir, abnormal pressure and reservoir-forming mechanism, etc., it is concluded that reservoir distribution in the Yanchang group in study area is mainly controlled by factors of “close source, low pressure, superior facies”
The data used to support the findings of this study are included within the article.
The authors declare that no conflicts of interest exit regarding the submission of this paper.
This study was supported by the Major national science and technology projects (no. 2017ZX05039001-005) and the Research project of Yanchang Oil Field Co., Ltd. (ycsy2020ky-B-10).