SEDIMENTARY CHARACTERISTICS OF LACUSTRINE DEEP-WATER GRAVITY FLOW AND ITS CONTROLLING EFFECTS ON RESERVOIRS: A CASE STUDY OF CHANG 6 OIL RESERVOIR IN HESHUI AREA, ORDOS BASIN, CHINA

Due to the transport capacity, Lacustrine gravity flow could carry a large amount of organic matter and shallow water sediments into deep lake areas, which promotes the formation of deep-water tight reservoirs and shale reservoirs. Therefore, the study of the origin, sedimentary characteristics and controlling effects on reservoirs of gravity flow is of great significance to the exploration of unconventional oil and gas in continental lake basins. Taking Chang 6 reservoir located in Heshui area of Ordos Basin as research object, this study applied a combination of field profile and core observation, microscopic identification, particle analysis, reservoir properties tests and other research methods, which discussed the formation mechanism, sedimentary features, sedimentary models of gravity flow and its controlling effects on reservoirs. According to study results, there are three types of gravity flow, slip-slump rocks, sandy debris flow and turbidity flow in researched area. Two types of gravity flow deposition models in this area have been identified considering the combination relationship of gravity flow types and sedimentary structures — slump olistoliths and sublacustrine fans. The slump olistoliths can be further divided into three microfacies including nearby slip-slump rocks, ligulate sandy-debris and distal turbidite lobes, while sublacustrine fans can be divided into middle fan and outer fan, whose subdivided microfacies are distributary channel, channel lateral margin, inter-channel and sheeted sand. The slump olistoliths and sublacustrine fans are respectively triggered by sudden geological events and seasonal floods. As indicated in particle analysis and reservoir properties tests, the sandy debris flow of middle fan distributary channel with high maturity of composition and texture, uniform particle size sorting and good reservoir properties has the best condition for oil and gas storage. Followed by turbidity flow of middle fan channel lateral margin and sheet sand of outer fan, the ligulate sandy-debris of slump olistoliths has certain degree of storage capacity. However, as the lithology and storage performance are highly heterogeneous and the reservoir space is discontinuous, the instantaneous sedimentary process leads to difficulty in large-scale reservoirs formation.