Abstract:

Controlled by tectonic movements, the shale subsidence-sedimentation center of the Sha 3 member of the Qingshui subsag in western Liaohe depression has a clear NW to SE migration trend. The (semi)-deep lake facies that are under-compensated for sedimentation also migrate synchronously. From the basinal center to land, the sedimentary types of deep lake-semi-deep lake-shallow lake-fan delta were developed during the Sha 3 member deposition in the Qingshui subsag, and the types I, Ⅱ and Ⅲ kerogen were formed, respectively. Accordingly, the brittle mineral (such as quartz and feldspar) contents in the shale increases gradually, while the clay contents decrease. This phenomenon reflects that the sedimentary facies control the organic geochemical conditions and mineral composition of the shale. Under the control of deep-major faults, sedimentary profile of the shale exhibits a wedge shape, which thins toward the west. The TOC gradually decreases from the maximum at the center, and the vitrinite reflectance (R_o) varies from 0.5% to 2.0%. All these geological conditions are favorable for shale oil and gas formation. At the end of the Dongying Formation deposition, the Sha 3 member in the Qingshui subsag has further subsided, and the thermal evolution maturity increases. Under the current burial depth, the shale is generally in the stage of hot mature oil generation. The burial depth can locally reach the highly matured stage and produced a large amount of natural gas. This makes shale gas as the main state 500 m ahead of time, forming a hydrocarbon generation pattern of “gas down and oil up”. The Sha 3 shale is mainly in stage B of the middle diagenetic stage, and a large number of lineated flaky illites occur. When the shale burial depth is over 4,000 meters, the ratio of illite-smectite mixed layer is below 15%. A variety of pore types is developed, and associated with clay minerals such as pyrite and illite. The shale porosity varies from 0.7% to 3.5% and pore diameter (8 nm to 35 nm) accounts for the largest total volume. The high-precision gas content test results show that the gas content of the Sha 3 shale in the Shuangxing 1 well is generally between 1.6 m³/t and 5.44 m³/t. With increasing diagenetic fractures in the shale reservoir, the shale gas content increases continuously. The shale gas content per unit of TOC increases with upwelling depth. Both the total gas and adsorbed gas contents have good linear relationships with TOC, but the increasing rate of total gas content is higher than that of the adsorbed gas. This is closely related to the thermal evolution of organic matter and presence of dissolved natural gas. Organic matter has a variety of types, and oil and gas symbiosis. The shale gas content is high, gas down and oil up, which formed a typical shale gas enrichment model of the Qingshui subsag.

Key words: shale gas, enrichment condition, third member of Shahejie Formation, Qingshui subsag, western depression