Quantitative evaluation of caprock sealing controlled by fault activity and hydrocarbon accumulation response: K gasfield in the Xihu Depression, East China Sea Basin

Abstract

Caprock-sealing capability affected by fault activity is an important problem that needs to be resolved in hydrocarbon accumulation analysis, especially for rifted basins. In this article, a quantitative model based on the mechanical properties estimated by geophysical logging was first established to characterize the brittle-ductile transition of mudstone caprocks. By establishing a burial depth reconstruction model, the brittle-ductile stage of the mudstone caprock at the termination time of fault activity (TTFA) was restored. Moreover, the caprock juxtaposition thickness (CJT) for brittle rock and the shale smear factor (SSF) for brittle-ductile rock were selected to quantitatively describe the sealing capacity of caprock controlled by faults. These mathematical methods were finally applied to a case study of the K gasfield in the Xihu Depression, East China Sea Basin. Compared with the sealing characterization based on the current mechanical properties, the brittle-ductile reconstruction at the TTFA and matching sealing evaluation better describe the effectiveness of the mudstone caprock. Additionally, the quantified sealing capacity and corresponding hydrocarbon response indicate that a CJT value of 56.1 m and an SSF value of 3.0 can be the threshold for caprock sealing and nonsealing in the study area. As a result, caprock with poor sealing led to vertical hydrocarbon leakage in the structural low zone and structural high zone, while strong sealing caprock located in the structural center zone effectively prevented the upward migration of hydrocarbons, thereby controlling the vertical hydrocarbon distribution in the K gasfield. In general, the evaluation accuracy of caprock sealing predominantly relies on the geological input to the model, especially the fluid pressure, rock mechanical parameters, and caprock and fault interpretation results, and it is also affected by further uncertainties arising from the sealing threshold definition and structural uplift.