页岩气成藏过程的阶段划分——以四川盆地宁西地区五峰组—龙马溪组页岩气成藏过程为例

四川盆地及其周缘地区普遍经历了多旋回构造演化过程,目前对于该区目标层系中页岩气多期活动和富集成藏规律与区域深埋藏—强隆升剥蚀作用过程具有何种相关性的研究相对薄弱。为此,以该盆地南部长宁页岩气区块西部（以下简称宁西地区）典型井为剖析对象,基于低温热年代学、流体包裹体和盆地热史模拟等综合研究,以上奥陶统五峰组—下志留统龙马溪组生烃动力学及其压力体系变化特征为载体,研究该区五峰组—龙马溪组新生代隆升与页岩气富集与破坏的耦合特征。研究结果表明：①该区磷灰石样品裂变径迹年龄为距今20~40 Ma、（U—Th）/He年龄为距今10~20 Ma,新生代地表剥蚀量为2 000 m;②五峰组—龙马溪组埋深沉降热史可划分为早志留世—晚三叠世低—中等成熟度、早—中侏罗世高成熟度和晚侏罗世—白垩纪过成熟度3个热演化阶段;③五峰组—龙马溪组方解石包裹体的流体均一温度呈双峰值特征、分别介于120~140 ℃和160~180 ℃,晚期流体盐度变化剧烈表明受到深部、浅表流体混染作用,保存条件被破坏;④五峰组—龙马溪组页岩生烃期长时间具有超压特征,晚侏罗世—白垩纪持续深埋藏使其生气速率达到高峰、形成异常高压,但新生代快速抬升剥蚀作用导致超压系统被破坏、现今为常压体系。结论认为,宁西地区五峰组—龙马溪组页岩气成藏过程具有明显的早期高压富集、深埋藏超高压保持、晚期构造隆升调整/破坏的多阶段特征。

Stage division of shale gas accumulation process: An example from the Wufeng Formation-Longmaxi Formation shale gas reservoir in the Ningxi area of the Sichuan Basin

The Sichuan Basin and its periphery have generally experienced a multi-cycle tectonic evolution process. At present, the correlation between the multi-stage activity and enrichment and accumulation laws of shale gas in the target strata of this area and the regional great burial depth-strong uplifting and denudation process is less researched. Taking the typical well in the western part of Changning Shale Gas Block (hereinafter referred to as Ningxi area)in the southern Sichuan Basin as the research object, this paper studied the coupling characteristics between the Cenozoic uplift and the shale gas enrichment and destruction in the Wufeng Formation-Longmaxi Formation of this area with the hydrocarbon generation dynamics of Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation and the change characteristics of its pressure system as the carrier, based on comprehensive studies, such as low-temperature thermochronology, fluid inclusion and basin thermal history simulation. And the following research results were obtained. First, apatite fission track ages (AFT) and (U-Th)/He ages are about 20-40 Ma and 10-20 Ma, respectively, indicating the Cenozoic surface denudation is 2000 m. Second, the burial depth and subsidence thermal history of Wufeng Formation-Longmaxi Formation can be divided into three thermal evolution stages, namely low-moderate maturity in the Early Silurian-Late Triassic, high maturity in the Early-Middle Jurassic, and over maturity in the Late Jurassic-Cretaceous. Third, the fluid homogenization temperature of calcite inclusion in the Wufeng Formation-Longmaxi Formation presents the characteristics of double peaks in the range of 120-140℃ and 160-180℃, respectively. The fluid salinity varies substantially in the late stage, indicating the preservation conditions are destroyed due to the hybridization of deep and near-surface fluids. Fourth, the Wufeng Formation-Longmaxi Formation shale presents the characteristics of overpressure for a long time in the period hydrocarbon generation, and the gas generation rate reaches the peak to form abnormal pressure due to the continuous great burial depth in the Late Jurassic-Cretaceous. In the Cenozoic, however, the overpressure system is destroyed by fast uplifting and denudation, and it is currently a normal pressure system. In conclusion, shale gas accumulation process in the Wufeng Formation-Longmaxi Formation of Ningxi area has obvious multi-stage characteristics, i.e., high-pressure enrichment in the early stage, great burial depth and ultrahigh pressure maintenance, and structure uplifting adjustment/destruction in the later stage.