Characteristics of pressure relief induced by shale brittle fracture in tectonic uplift area and its influence on shale oil enrichment: a case study of Chang 73 sub-member of Yanchang Formation in Yan'an area

Abstract

In order to gain a deeper understanding of the pressure relief characteristics of shale brittle fracture caused by large-scale tectonic uplift and its impact on shale oil enrichment, this paper, taking the Chang 73 sub-member of Yanchang Formation in Yan'an area of Ordos Basin as the research object, quantitatively analyzes the evolution of different lithological properties from overpressure to underpressure or near normal pressure, and further summarizes its influence on the enrichment of shale oil by utilizing the modified overpressure identification plate and numerical simulation techniques and taking into account the effects of temperature reduction, elastic rebound, and brittle rupture of overpressured shale in the process of tectonic uplift. The results show that the origins of overpressure at the end of the Early Cretaceous in the Chang 73 sub-member in the study area are mainly hydrocarbon generation and disequilibrium compaction, and the overpressure of the sandstone layer mainly originates from the overpressure transference from the neighboring source rocks; elastic rebound and temperature reduction due to tectonic uplift since the Late Cretaceous dominates the formation of underpressure in the Chang 73 sub-member sandstones in the study area, and brittle fracture pressure relief and temperature reduction lead to the formation of weak overpressure or near normal pressure in the Chang 73 sub-member shale. The timing of brittle fracture pressure relief of the top and bottom sets of organic-rich shale in the Chang 73 sub-member of the study area is not synchronized, resulting in large transport dynamics (source-reservoir excess pressure difference) between the top and bottom sets of organic-rich shale and their intercalated sandstones with magnitudes ranging from 16 to 22 MPa. Meanwhile, fractures in the top shale have closed during oil and gas preservation, and there is an excess pressure differential of about 1.42-6.80 MPa between it and the underlying sandstone, which strengthens the hydrocarbon containment capacity of the shale. Differential evolution of formation pressure under different source-reservoir configurations plays an important role in controlling shale oil enrichment.

Keywords

• tectonic uplift
• brittle fracture
• elastic rebound
• shale oil
• yanchang formation
• triassic
• yan'an area
• ordos basin