Intra-layer heterogeneity of sandstone with different origins in deep-water environment and its causes

Abstract In recent years, a lot of analysis and discussions on the origins, flow mechanisms, and sedimentation models of deep-water sandstone have been conducted. However, the intra-layer heterogeneity of different sandstone lacks of study. Intra-layer heterogeneity refers to the change of vertical reservoir characteristics within a single sandstone. The degree of change is often the internal cause of intra-layer contradictions in oil and gas production. After analyzing a large number of the core samples and well logging data of the Chang 6 reservoir in the Zhengning area, southern Ordos Basin in central China, it is believed that the Chang 6 reservoir in the study area is of deep-water sediments. The sandstone's origins include sandy debris flow, sandy slumping and turbidity current. Based on the statistical data of the 554 physical property tests, a comprehensive evaluation of the heterogeneity within the study area was conducted using the BP neural network method for the first time, and the test accuracy rate reached 89.47%. It is considered that the intra-layer heterogeneity of the three types sandstone from weak to strong is sandy debris flow, sandy slumping and turbidity current. After an in-depth analysis of sedimentation, it is believed that the main factors affecting the intra-layer heterogeneity of different deep water environment sandstones are sedimentation processes, granular rhythm and bedding structure. A combination of qualitative and quantitative studies on these three influencing factors were conducted. It is considered that laminar flow, coarser grain size, homogeneous rhythm and massive structure make the weak intra-layer heterogeneity of sandy debris flow. While, laminar flow, finer granularity, homogeneous rhythm and abundant stratified structure forming a moderate intra-layer heterogeneity of sandy slumping. Finally, turbulent suspension, the dominant features of the positive rhythm, followed by the reverse rhythm, and the laminar structure, followed by the stratified structure, resulted in a strong intra-layer heterogeneity of the turbidity current.