Effects of the fault-sandbody configuration evolution model on differential enrichment of far-source tight sand gas: A case study of the Jurassic Shaximiao Formation, central Sichuan Basin

This study aims to determine the differential enrichment patterns of far-source tight sand gas in the Jurassic Shaximiao Formation, central Sichuan Basin. By conducting seismic interpretation of source rock-rooted faults, characterizing sand bodies, and reconstructing paleostructures during hydrocarbon accumulation, we analyze the fault-sandbody configuration modes in the Shaximiao Formation both during the hydrocarbon accumulation period and in the present day. Furthermore, the evolutionary model and spatial distribution of the configuration from the hydrocarbon accumulation period to the present are identified. Based on these, as well as the distribution characteristics of current gas producers, we explore the influence of the evolutionary model on differential natural gas enrichment. The results reveal the presence of three distinct modes of fault-sandbody configuration in the Shaximiao Formation, that is, juxtaposition of fault against near-horizontal sandbodies (mode 1); juxtaposition of fault against gently dipping sandbodies with consistent dip directions (mode 2); and juxtaposition of fault against gently dipping sandbodies with opposite dip directions (mode 3). From the hydrocarbon accumulation period to the present, the fault-sandbody configuration modes have experienced three types of evolutionary patterns: inherited (type Ⅰ), adjusted (type Ⅱ), and reversed (type Ⅲ). Specifically, type Ⅰ can be further divided into type Ⅰ₁ (mode 3 persisting both during the hydrocarbon accumulation period and in the present day) and type Ⅰ₂ (mode 1 persisting during both periods). Type Ⅱ is characterized by a transition from mode 1 during the hydrocarbon accumulation period to mode 3 at present, while type Ⅲ represents a shift from mode 3 during the hydrocarbon accumulation period to mode 2 in the present day. These three types of evolutionary patterns exert different influences on natural gas enrichment. Specifically, the type Ⅰ₁ inherited evolutionary pattern enables natural gas to accumulate primarily in the hanging wall blocks of faults and remain preserved to the present day. Consequently, most gas producers located in these blocks show high or moderate productivity. In this evolutionary pattern, only a small amount of natural gas accumulates in the footwall blocks of faults and remains preserved to date, leading to low productivity in most wells in these blocks. The type Ⅰ₂ inherited evolutionary pattern allows natural gas to accumulate and remain preserved to date primarily in both the hanging wall and footwall blocks, with most gas producers in these blocks showing moderate or low productivity. The type Ⅱ adjusted evolutionary pattern facilitates the accumulation, adjustment, and modification of natural gas in both the hanging wall and footwall blocks. Consequently, natural gas may further accumulate or get enriched in the hanging wall blocks, where gas producers show high, moderate, or low productivity. In contrast, natural gas may escape from the footwall blocks after adjustment, resulting in low or ultra-low productivity in most gas producers in the footwall blocks. The type Ⅲ reversed evolutionary pattern leads to natural gas enrichment in the hanging wall blocks but dissipation to date due to reversal, with only a small amount of natural gas accumulating in the footwall blocks and reaccumulating there to date after reversal. In this pattern, most gas producers in the hanging wall and footwall blocks show low or ultra-low productivity. The Jinqiu block, located in the northern part of the central Sichuan Basin, contains two types of natural gas accumulation: (1) natural gas accumulation in the hanging wall blocks, characterized by inherited preservation and enrichment, under the type Ⅰ₁ inherited evolutionary pattern; and (2) natural gas accumulation in the hanging wall blocks, featuring adjustment, modification, and enrichment, under the type Ⅱ adjusted evolutionary pattern. Therefore, this block be prioritized as a key target area for future exploration of far-source tight sand gas in the Shaximiao Formation.