Caprock characteristics and sealing evaluation of Pingbei gentle slope belt in Xihu Sag
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摘要:
盖层是西湖凹陷平湖构造带平北缓坡带的主要控藏因素之一。为厘清平北缓坡带优质盖层分布及控藏规律，利用该区满覆盖1 200 km2三维地震和23口探井资料，运用井约束地震反演技术，分析了盖层发育规律及封堵效果。结果发现，平北缓坡带平上段存在一套平均孔隙度1.5%的泥岩区域盖层S1，致使平湖组烃源岩生成的油气能够绝大部分滞留于平湖组内部。选取T31界面上下50 ms时窗制作了S1盖层时间厚度图，较为客观地展现了这套盖层的全貌及分布特征，并指出L1、W4、B2等钻井失利是由于局部封堵性变差所致。考虑到断层对盖层封堵性的影响，认为当泥质涂抹因子＜1.8时才对油气具有良好封堵性。综合考虑盖层厚度、泥质含量、断裂等因素，将平北缓坡带划分为完全封盖区、部分封盖区、无能力封盖区，以期有助于该区的油气勘探。
Abstract:Caprock is one of the main reservoir-controlling factors of the Pingbei gentle slope belt (PGSB) in the Pinghu structural belt of the Xihu Sag. To find out the distribution law of high-quality caprocks and its role in reservoir control in the PGSB, 3D seismic data covering an area of 1200 km2 and 23 exploratory wells in the PGSB were analyzed by well-constrained seismic inversion technology, and the development of the caprock in PGSB was analyzed. Results show that there is a set of regional mudstone caprock S1 on average porosity of 1.5% in the upper Pinghu section of the PGS, thus most of the oil and gas generated from the source rocks of the Pinghu Formation could be retained in the Pinghu Formation. The time window of the upper and lower 50 ms of the T31 interface was selected to produce the time thickness map of the S1 caprock, which objectively showed the overall appearance and distribution characteristics of the caprock, and pointed out that the poor local sealability caused missing of the caprock in wells of L1, T2, W4, B2, etc.. Meanwhile, considering the influence of faults on the sealability of the caprock, it was pointed out that when mud smear factor was less than 1.8, it had good sealability for oil and gas. Overall consideration of caprock thickness, shale content, faults, and other factors, the sealability of the S1 caprock in the PSG belt could be classified and evaluated for providing guidance to oil and gas exploration in the area in future.
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