Shuo Chen, Renhai Pu, Huiqiong Li, Hongjun Qu, Tianyu Ji, Siyu Su, Yunwen Guan, Hui Zhang. Distribution characteristics of delta reservoirs reshaped by bottom currents: A case study from the second member of the Yinggehai Formation in the DF1-1 Gas Field, Yinggehai Basin, South China Sea[J]. Acta Oceanologica Sinica.
Citation:
Shuo Chen, Renhai Pu, Huiqiong Li, Hongjun Qu, Tianyu Ji, Siyu Su, Yunwen Guan, Hui Zhang. Distribution characteristics of delta reservoirs reshaped by bottom currents: A case study from the second member of the Yinggehai Formation in the DF1-1 Gas Field, Yinggehai Basin, South China Sea[J]. Acta Oceanologica Sinica.
Shuo Chen, Renhai Pu, Huiqiong Li, Hongjun Qu, Tianyu Ji, Siyu Su, Yunwen Guan, Hui Zhang. Distribution characteristics of delta reservoirs reshaped by bottom currents: A case study from the second member of the Yinggehai Formation in the DF1-1 Gas Field, Yinggehai Basin, South China Sea[J]. Acta Oceanologica Sinica.
Citation:
Shuo Chen, Renhai Pu, Huiqiong Li, Hongjun Qu, Tianyu Ji, Siyu Su, Yunwen Guan, Hui Zhang. Distribution characteristics of delta reservoirs reshaped by bottom currents: A case study from the second member of the Yinggehai Formation in the DF1-1 Gas Field, Yinggehai Basin, South China Sea[J]. Acta Oceanologica Sinica.
Distribution characteristics of delta reservoirs reshaped by bottom currents: A case study from the second member of the Yinggehai Formation in the DF1-1 Gas Field, Yinggehai Basin, South China Sea
State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China
2.
Research Institute of Yanchang Petroleum Group, Xi’an 710065, China
3.
Research Institute of Petroleum Exploration and Development, Beijing 100083, China
4.
Research Institute of Zhanjiang Branch of CNOOC, Zhanjiang 524057, China
Funds:
The National Natural Science Foundation of China’s major project “Research on Geophysical Theories and Methods of Unconventional Oil and Gas Exploration and Development”, Task I: “China’s Tight Oil and Gas Reservoir Geological Characteristics, Classification and Typical Geological Model Establishment” under contract No. 41390451.
The Dongfang1-1 gas field (DF1-1) in the Yinggehai Basin (YGHB) is currently the largest offshore self-developed gas field in China and is rich in oil and gas resources. The second member of the Pliocene Yinggehai Formation (YGHF) is the main gas-producing formation and is composed of various sedimentary types; however, a clear understanding of the sedimentary types and development patterns is lacking. Here, typical lithofacies, logging facies and seismic facies types and characteristics of the YGHF are identified based on high-precision 3D seismic data combined with drilling, logging, analysis and testing data. Based on 3D seismic interpretation and attribute analysis, the origin of high-amplitude reflections is clarified, and the main types and evolution characteristics of sedimentary facies are identified. Taking gas formation IIU as an example, the plane distribution of the delta front and bottom current channel is determined; finally, a comprehensive sedimentary model of the YGHF second member is established. This second member is a shallowly buried “bright spot” gas reservoir with weak compaction. The velocity of sandstone is slightly lower than that of mudstone, and the reflection has medium amplitude when there is no gas. The velocity of sandstone decreases considerably after gas accumulation, resulting in an increase in the wave impedance difference and high-amplitude (bright spot) reflection between sandstone and mudstone; the range of high amplitudes is consistent with that of gas-bearing traps. The distribution of gas reservoirs is obviously controlled by dome-shaped diapir structural traps, and diapir faults are channels through which natural gas from underlying Miocene source rocks can enter traps. The study area is a delta front deposit developed on a shallow sea shelf. The lithologies of the reservoir are mainly composed of very fine sand and coarse silt, and a variety of sedimentary structural types reflect a shallow sea delta environment; upward thickening funnel type, strong toothed bell type and toothed funnel type logging facies are developed. In total, 4 stages of delta front sand bodies (corresponding to progradational reflection seismic facies) derived from the Red and Blue Rivers in Vietnam have developed in the second member of the YGHF; these sand bodies are dated to 1.5 Ma and correspond to four gas formations. During sedimentation, many bottom current channels (corresponding to channel fill seismic facies) formed, which interacted with the superposed progradational reflections. When the provenance supply was strong in the northwest, the area was dominated by a large set of delta front deposits. In the period of relative sea level rise, surface bottom currents parallel to the coastline were dominant, and undercutting erosion was obvious, forming multistage superimposed erosion troughs. Three large bottom current channels that developed in the late sedimentary period of gas formation IIU are the most typical.
Figure 1. Regional location and Cenozoic strata comprehensive histogram of the study area. a. Bathymetric map showing the location and geological context of the DF1-1 gas field in the Yinggehai Basin, northwestern margin of South China Sea. b. Time thickness map of the second member of YGHF in the DF1-1 gas field (gas formations I–IIIU), showing a gradually decreasing stratigraphic thickness from northwest to southeast, with an obvious step-like pattern of decline, which also indicates that the provenance of the study area was from the northwest direction during the deposition periods of the four gas formations. c. The comprehensive histogram of stratigraphic and sedimentary facies shows the changes in lithology and sedimentary facies of the sedimentary strata of the YGHB in the northern SCS since the Cenozoic against the background of four major tectonic evolution stages. SCS: South China Sea; YGHB: Yinggehai Basin; QDNB: Qiongdongnan Basin; ZRMB: Zhujiang River Mouth Basin; Q: Quaternary; N1: Early Neogene; N2: Late Neogene; E2: Middle Paleogene; E3: Late Paleogene; Mz: Mesozoic basement.
Figure 2. Content of planktonic foraminifera, paleowater depth curve and horizon calibration of Well DF1-1-11. The relationship between the percentage of planktonic foraminifera (P/%) and paleobath (D/m) in the western SCS is according to Li et al. (2004). The paleo-water depth curve is according to He et al. (2011). Well 11 is located in the middle of the gas field, and its location is shown in Fig. 1b. The results show that gas formations I–IIIU of Well 11 in the DF1-1 gas field correspond to the early stage of the VII N21 zone, and the paleowater depth is approximately 25–45 m. GR: natural gamma ray.
Figure 3. Typical logging sequence and seismic horizon framework of the second member of the YGHF in the YGHB. a. Sequence division and stratigraphic histogram. b. Seismic cross-section of wells 15-5-4 in the SE direction. c. Seismic cross-section of Well 9 in the WE direction. See Fig. 1b for the locations. TWT: two-way-travel time; GR: natural gamma ray.
Figure 4. Lithological characteristics of cores in the second member of YGHF in the DF1-1 gas field, YGHB. a. Light gray fine siltstone, Well 3 gas formation IIL, 1 297.5 m. b. Reddish brown argillaceous siltstone, Well 3 gas formation IIIU, 1 334.2 m. c. Gray siltstone, Well 5 gas formation IIU, 1 415.6 m. d. Dark gray argillaceous siltstone (containing biological trace fossils), Well 5 gas formation I, 1 347.6 m. e. Argillaceous siltstone in the upper part, siltstone in the middle and lower parts, Well 5 gas formation IIU, 1 390.94–1 395.95 m. The probability particle size curves (f) show the traction flow characteristics of “two segments” and “three segments”.
Figure 5. Sedimentary structure types of the second member of YGHF in the DF1-1 gas field, YGHB. a. Fine sandstone-siltstone with interlaminar fractures. b. Massive argillaceous siltstone with mud clasts, where a large number of biological disturbance occur. c. Massive siltstone with biological burrows. d. Shallow sea storm sedimentary sequence, with section b showing parallel bedding in the lower part and section c showing convolute bedding in the upper part. e. Rhythmic bedding. f. Flaser bedding.
Figure 6. Logging facies type (a) and core sketch (b) of the second member of the YGHF in representative Well 5 in the western part of the DF1-1 gas field. Several reverse cycle parasequence-related logging facies types are mainly developed, and a large number of interlaminar fractures are developed in fine sandstone-siltstone cores. GR: natural gamma ray; ILM: resistivity of medium investigated induction log; ILD: resistivity of deep investigated induction log; CNL: compensated neutron log; DEN: compensated density log.
Figure 7. Logging facies type (a) and core sketch (b) of the second member of the YGHF in representative Well 3 in the eastern part of the DF1-1 gas field. Several parasequence-related toothed bell-shaped or toothed funnel-shaped logging facies are mainly developed. Cores contain mud clasts, and a large number of biological disturbance characteristics can be seen. GR: natural gamma ray; ILM: resistivity of medium investigated induction log; ILD: resistivity of deep investigated induction log; CNL: compensated neutron log; DEN: density log.
Figure 8. Typical seismic facies types and possible geological genesis of the second member of the YGHF in the DF1-1 gas field 3D seismic survey.
Figure 9. Seismic sections (a), sedimentary facies (b) and attenuation anomaly (c) of wells 9-4-2-3 in the NE direction in the second member of the YGHF in the DF1-1 gas field 3D seismic survey. See Fig. 10 AA' for the location. GR: natural gamma ray; RILD: resistivity of deep investigated induction log; AC: acoustic.
Figure 10. Plane distribution maps of the top surface structure in the time domain (a), time thickness (b), root mean square (RMS) amplitude (c) and generalized low-frequency shadow attenuation (d) of gas formation IIU in the DF1-1 gas field. See Fig. 9 for profile AA' and Fig. 11 for profile BB'.
Figure 11. Seismic facies evolution analysis of the second member of the YGHF in the cross-sections of wells 5-11-3 in the NE direction by flattening the top interfaces. a–d correspond to each deposition period of gas formations I–IIIU. See Fig. 10 BB' for the profile position.
Figure 12. Plane distribution characteristics of delta front sand bodies and bottom current erosion channels of gas formation IIU in the second member of the YGHF in the Dongfang 1-1 gas field, Yinggehai Basin. a. Plane distribution map of seismic facies. b. Plane distribution map of sedimentary subfacies. It is inferred that the cause of the shallow-water bottom current is similar to that of the modern surface current, the flow direction is affected by the East Asian monsoon, the red arrow is clockwise in summer and the pink arrow is counterclockwise in winter.
Figure 13. Comparison of U-Pb ages of detrital zircon from the second member of the YGHF in the DF gas field, YGHB, with the age spectra of modern river sediments in the three potential provenance areas of the DF area. a–c are the peak age characteristics of provenances in the eastern, northwestern, and western parts of the study area, respectively (according to Wang et al. (2014, 2015, 2019)); d–f are the U-Pb age distribution characteristics of the Q3-29, Q9-01 and Q5-17 samples in the study area, and the red arrows are the marker age peak of each potential source area.
Figure 14. CL diagram of zircon cathodoluminescence in the second member of the YGHF in the DF gas field, YGHB, where 206Pb/238U and 1σ corresponding ages are taken for measuring points less than 1000 Ma, and the corresponding ages of 207Pb/206Pb and 1σ are taken for dating points greater than 1000 Ma
Figure 15. Comprehensive sedimentary model of the second member of the YGHF in the DF1-1 gas field, YGHB.
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