辽河坳陷清水洼陷沙三段页岩气富集条件

受构造作用控制,辽河西部凹陷清水洼陷沙三段页岩沉降-沉积中心发生由北西向南东方向的明显迁移,表现为欠补偿沉积的半深湖-深湖相同步迁移。从湖盆中心向陆方向,清水洼陷沙三段沉积时期主要发育了深湖-半深湖-浅湖-扇三角洲等沉积类型,对应形成了从Ⅰ、Ⅱ到Ⅲ型的干酪根分布。与此对应,页岩中的石英、长石等脆性矿物含量递次增多,而黏土矿物含量逐渐减少,反映为沉积相对页岩有机地球化学条件和矿物成分的控制作用。在深大断裂控制下,页岩沉积剖面表现为向西减薄的楔形,TOC从中心处的最大值向外逐渐降低,成熟度R_o介于0.5%~2.0%,为页岩油气的形成提供了有利的富集条件。在现今埋深状态下,洼陷区页岩一般处于热成熟生油阶段,埋深较大的局部地区可达高成熟阶段,进入页岩气为主状态,形成“下气上油”的生烃格局。沙三段页岩主要处于中成岩阶段B期,出现了大量定向排列的片状伊利石。在4 000 m以深的页岩中,伊蒙混层比小于15%,发育了多种孔隙类型,伴生黄铁矿和伊利石等黏土矿物,孔隙度一般为0.7%~3.5%,8~35 nm的页岩孔隙直径所占体积最大。高精度含气量解析结果表明,双兴1井沙三段页岩含气量为1.6~5.44 m³/t,随着页岩储层成岩裂缝增加,页岩含气量不断增加。总含气量、吸附含气量与TOC具有很好的相关性,但总含气量增加速度较吸附气增量速度更大,这与有机质生油气产物关系密切,与溶解态天然气的存在也不无关系。有机质类型多样、油气共生、含气量较大且下气上油,形成了辽河清水洼陷特色的页岩气富集模式。

Shale Gas Enrichment Conditions in the Third Member of Shahejie Formation in Qingshui Subsag,Liaohe Depression

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.