Sedimentary Characteristics And Reservoir Modelling Of Shallow-marine Gravity Flow Deposition From Huangliu Formation In Dongfang Area, Yinggehai Basin, Northwestern South China Sea

The reservoir in upper Miocene Huangliu formation of Yinggehai Basin is shallow-marine gravity flow deposits with burial depth from 2600m to 3500m. The overpressure begin to appear at ～2500m in Dongfang area, the member #1 of Huangliu formation is under overpressure environment. The average pressure coefficient of gas field DF13-1 and DF13-2 is 1.92 and 1.76, respectively. The previous studies of gravity flow are mainly focus on deep-water environment, including Gulf of Mexico, Angola, Niger, Amazon, Indus and South China Sea. Compared with deep-water gravity flow, it is shortage of studies for sedimentary system in shallow-marine environment due to its general untypicalness and small-scale. Thus it is important to strengthen the study of depositional background, sedimentary characteristics, sedimentary model and genetic mechanism. Based on the 3D seismic data, well logging, core and test data, this dissertarion aim to (1) establish the isochronous stratigraphic framework combined with seismic-logging technique; (2) analyze the depositional architecture and evolution of lobes using seismic sedimentology; (3) analyze the characteristics of sedimentary facies distribution and controlling factors of depostiton for shallow-marine gravity flow, and establish the sedimentray model; (4) analyze the connectivity and anisotropy of thick sandstone, then summarize the stacking patterns and controlling factors of reservoir quality for thick sandstone; (5) construct the 3D reservoir model of Huangliu formation in study area. The main conclusions are as follows:The Sr/Ba ratios, "adjusted B" and paleosalinity of sandstone samples from core interval indicate brackish water environment. The V/Ni ratios and Ce anomalies of sandstone samples from core interval indicate weak reduction environment. The paleo-waterdepth calculated by trace elements ranges from 72m to 202m with average of 135m. In addition, the characteristics of trace fossils, foraminifer, calcareous nannofossil and algae-spore ratios also suggest that the paleo-waterdepth should less than 200m, probably outer neritic environment.Based on seismic facies, RMS-amplitude and logging, five depositional elements are recognized, including sandy debris flow channel, turbidity channel, sand-rich lobe and mud-rich lobe. The sandy debris-flow channel which is general inside the lobe shows highly-incised and low RMS-amplitude reflection with width of 1-2km. The core interval of sandy debris-flow channel which is composed of massive fine-grained sandstone with mudstone clasts and planar fabric comprises multiple coarsening-upward units. The turbidity channels are characterized by low or high RMS amplitude on the plane and worm-shaped in profile view with width of lea than 0.5km. A small size of frontal splays is commonly developed in the front of turbidity channel, thus forming lobe-channel-lobe depositional style. The core interval from turbidity channel is composed of siltstone with graded bedding and parallel bedding which indicate multiple Ta-Tb division of Bouma sequence. Sand-rich lobe and much-rich lobe which show continuous sheet configure are characterized by high RMS amplitude and low RMS amplitude, respectively.The evolution of large-scale, medium-scale and small-scale lobes are analyzed using seismic tratigraphic and seismic sedimentology methods. The lobes from gas group # Ⅰ and Ⅱ are divided into 4 large-scale,6 medium-scale and 9 small-scale lobes according to superimposition of seismic event, intensity of channel incision and stratal slicing, respectively. Meanwhile the 6 medium-scale lobes correspond to 6 layers of gas group # Ⅰ and Ⅱ. Thus the isochronous stratigraphic framework has been established, and the cyclity of sandstone geochemistry corresponds well to the interface of gas group # Ⅰ and Ⅱ.Based on the detailed description of core interval,10 lithofacies have been recognized, including massive fine-grained sandstone and siltstone (Sm), mud clasts-rich fine-grained sandstone (Sc), parallel bedding fine-grained sandstone (Sp), horizontal bedding siltstone (Sh), horizontal bedding argillaceous siltstone (Ah), wavy bedding argillaceous siltstone (Sw), deformed bedding siltstone (Sd). ryhthem bedding fine-grained sandstone (Sr), biotubation argillaceous siltstone (Ab) and horizontal bedding silty mudstone (Mh). The logging facies include box-type, bell-type, funnel-type, finger-type and straight-type. Based on the detailed analysis of lithfacies and logging facies, two sedimentary systems, including 8 sedimentary microfacies are recognized. The neritic system is composed of neritic mud microfacies and neritic sandbar microfacies. The shallow-marine gavity flow sedimentary system is composed of gully, main channel, distributary channel, crevasse channel, overbank and sheet sand microfacies.The sandstone of Huangliu formation is main composed of lithic quartze sandstone and felsaspar lithic sandstone, few sandstone are subarkose and quartz sandstone. Western sourced sandstones form the shallow-sea gravity flow which are mainly sublitharenite having high feldspar (average is 6.1%) and lithic content (average is 11.7%), low shale content (average is 3.8%). However, eastern sourced sandstones, which are mainly subarkose-quartz siltstone having low feldspar and lithic content with averages of 4.6% and 2.7%, respectively, high compositional maturity, high shale content (average is 18.6%), form the neritic sand bar. The type of diagenesis in study area includes compaction, cementation and dissolution. Compaction and cementation are destructive diagenesis, however, dissolution can improve the reservoir physical to some extent. Generally, the intensity of compaction is medium in study area, the detrital grains show point contact to line contact. The common cements in study area are calcite or ferrocalcite, dolomite or ankerite, clay minerals, and few siliceous cement, siderite and pyrite. The calcite or ferrocalcite occurs as pore-filling or replace other detrital grains. The clay minerals include chlorite, kaolinite and illite. In addition, the CO2-rich thermal fluid flow into pore space though fault system, resulting in the dissolution of feldspar, cements and biogenic body, e.g., the average ratio of secondary pore is 43.3%. The intense dissolution induce the increase of pore-throat radius and improve of reservoir physical. The porosity and permeability of Huangliu formation range from 6.2-22.7% with average of 17.2% and 0.1-344.6md with average of 17.1md, respectively. In general, the reservoir physical is characterized by medium porosity, low permeability and heterogeneous pore structure.Based on the adjacent river system, seismic reflection, heavy mineral analysis, paleo-current, and sandstone geochemistry, we think that:(1) the western KunTum uplift provide the source materials for the gavity flow sandstone; (2) The shallow-sea gravity flow sandstones exhibit low zircon, tourmaline content, and high magnetite, garnet content, while the neritic sandbar sandstones exhibit high zircon, tourmaline and leucosphenite content, and low magnetite, garnet content. The direction of paleo-current in study area is along the direction from southeastward to eastward; (3) The similar rare earth element (REE) patterns of sandstone samples from three western wells indicate that they are sourced from the same provenance. The diagrams of Th-Sc and Co/Th-La/Sc and elemental ratios of these sandstone samples show that they were derived from intermediate to felsic source rocks. The measured geochemistry data of these core samples fall into the fields of active continental margin or continental island arc in the diagram of La-Th-Sc, Th-Sc-Zr/10, Ti/Zr-La/Sc and major element discrimination, indicating that the source rocks of the shallow-sea gravity flow sandstones were formed under the tectonic setting of an extrusion environment.The sandstone thickness varies dramatically laterally, and shows heterogeneous vertically. In each gas field DF13-1 and DF13-2, the sandstone shows relatively better lateral connectivity. However, the sandbody between almost disconnect to each other. The distribution of sedimentary facies is complex. In DF13-1 gas field, the main channel is relatively more developed. However, the channel and overbank are relatively more developed in DF13-2 gas field. By integrating the distribution of sedimentary facies and sandbody, we think that the controlling factors of shallow-marine gravity flow deposition are the abundant source materials, relative sea-level change and paleotopography. Firstly, the large-scale Blue river can provide mass source materials which main affect Yingxi slope zone and Dongfang area, and the progradation reflection is obvious from Yingxi slope to Dongfang area; the lobe scale is decreaing from layer# 1 to layer# 6, indicating that the rising sea-level can reduce the energy of gravity flow, resulting in worse pore structure and reservoir physical; in addition, the thick sandstone is main distributed in low-lying area, which indicate that the heterogeneous of thick sandstone is main controlled by paleotopography.The channel sandstones are further divided into four types, including erosion-filling, incised-filling, incised-reworked and cutting-overlay type. The erosion-filling channel which is characterized by well-sorted fine-grained sandstone and low shale content, is commonly located at the proximal of lobe. The seismic reflection of erosion-filling channel shows U-shaped, and the logging shows box-type with commonly single sandbody thickness greater than 25m. The incised-filling channel which is characterized well-sorted fine-grained sandstone and siltstone and low shale content, is commonly located downstream of erosion-filling channel. The seismic reflection of incise-filling channel shows narrow V-shaped with commonly single sandbody thickness less than 25m. The incised-reworked channel which is composed of silt-finestone, is reworked by bottom-current. The seismic reflection of incised-reworked channel shows wide V-shaped with commonly single sandbody thickness greater than 20m. The cutting-overlay channel which is composed of siltstone, is commonly located at distal of lobe. The seismic reflection of cutting-overlay channel shows stripped-shaped in plan view and worm-shaped in profile with single sandbody thickness between 15m-20m. Based on the study of genetic channel sandstone, we conclude four types of superimposition, including erosion-filling type, incise-filling type, incise-reworked type, cutting-overlay type and superpositioned sheet sandstone. Meanwhile, the sandbody architecture controls the gas-water distribution model. In profile, each sandbody has a uniqe gas-water distribution model, thus forming the lithologic reservoir.The grain size and permeability show composite reverse rhythm, however, there is little difference of porosity in profile. The permeability in upper part is higher than that in lower part. There is big difference of permeability between upper part and lower part in erosion-filling channel, incise-filling channel and cutting-overlay channel; but there is little difference of permeability and flw units of sheet sandstone due to its thinner thickness.The reservoir quality is mainly controlled by sedimentary microfacies. The main reservoir is channel sandstones with large thickness, wide distribution and better physical properties. However, the incised-reworked sandstones show the best reservoir quality with the average porosity and permeability of 19% and 99.07md, respectively. The main diagenetic factor for porosity reduction is compaction, the percentage of primary porosity reduction through compaction and cementation is 60% and 20%, respectively. The physical properties in middle and upper part of single sandbody increase with the increase of pressure coefficient. The thermal fluid in mud diapir zone can obviously improve the reservoir quality. On the one hand, the overpressure can inhibit mechanical compaction and the content of carbonate cement. On the other hand, the CO2-riched thermal fluid which migrated through fracture system can dissolve the unstable minerals, resulting the increase of secondary pores. The submarine fan sandstones sourced from the west form the lithology-controlled reservoir which is located in the west of mud diapir. The pressure coefficient decreases with the increase of distance between wells and mud diapir, resulting the decrease of porosity, decrease of secondary pores ratio and increase of carbonate cement content. The neritic sand bar sourced from the east form the diapir-controlled reservoir which is located in the east of mud diapir, it is characterized by tight lithology and small-scale of gas reservoir.This study integrates seismic-well data and depositional architecture to establish the fine sedimentary facies and lithofacies model. The reservoir physical model is based on sedimentary facies and flow units model, which is better than the results just using stochastic modelling.