Natural Gas Geochemistry of Jurassic Shaximiao Formation in the Western Sichuan Basin, China: Fault-Controlled Differentiation in Accumulation Process

Abstract

The Jurassic Shaximiao Formation in western Sichuan has made significant contributions to the natural gas reserves and production of China, with many gas fields successively discovered. The accumulation evolution progress and discrepant gathering law are continuously improved with the constantly deepened exploration and development. In order to find evidence for the difference in gas accumulation and the method to reshape the migration and accumulation of natural gas and their dynamic processes based on time and space, this study analyzes the dynamic accumulation process of the Middle Jurassic Shaximiao reservoir in the Zhongjiang gas field using the geochemical parameters, including the conventional composition of natural gas, light hydrocarbon components, carbon isotope, and formation water, of more than 160 natural gas samples from 155 wells. The results show that the natural gas-filling period, migration patterns, and phase state, etc., of the Shaximiao Formation are closely related to the distance between the hydrocarbon source faults. The geochemistry tracer parameters of natural gas and formation water reveal abundant information about the accumulation process. The Sha-3 gas reservoir, featuring a two-period hydrocarbon supply, has two migration patterns, i.e., seepage and diffusion, dominated by the water-soluble phase. Similarly, the Sha-2 gas reservoir, featuring a late single hydrocarbon supply, has the same migration patterns, dominated by the miscible phase of the water-soluble phase and free phase. The main migration pattern of the Sha-1 reservoir, which features a late single hydrocarbon supply, is seepage, dominated by the free phase. From the lower part of the Sand-3 gas reservoir to the upper part of the Sha-1 gas reservoir, the natural gas exhibits a trend where the early–late continuous filling turns into the late single filling, the lateral migration distance of oil and gas becomes shorter gradually, and the gas-rich range of the gas reservoir decreases gradually. The results of this study can provide a valuable reference for gas reservoirs controlled by faults and river sand bodies.

Gas–Source Comparison and Hydrocarbon Generation and Expulsion Histories of the Main Source Rocks of the Middle Jurassic Shaximiao Formation, Western Sichuan Basin

“…The natural gas generated at this stage migrated along faults to reach the reservoirs within the Shaximiao Formation, forming the ancient gas reservoir within the Shaximiao Formation. Overlying Suining Formation and Penglaizhen Formation developed substantial layers of thick mudstone caprocks, which exhibited excellent sealing properties and established favorable conditions for the preservation of natural gas . (3) Since the end of the Early Cretaceous, the Western Sichuan Basin has underwent uplift and erosion due to the impact of the Yanshan–Himalayan movement .…”

“…Due to the development of hidden faults and microfractures in the buried layers in the Western Sichuan Basin, coupled with the good permeability of the sandstone, there are no large-scale faults . And the thick mudstone overlying the Suining Formation is used as the effective seals of the region . Combined with the distribution of high-quality reservoirs within the Shaximiao Formation, − the recommended exploration prospects in the study region are situated primarily in the central and southern regions (Figure ).…”

“…Overlying Suining Formation and Penglaizhen Formation developed substantial layers of thick mudstone caprocks, which exhibited excellent sealing properties and established favorable conditions for the preservation of natural gas. 28 (3) Since the end of the Early Cretaceous, the Western Sichuan Basin has underwent uplift and erosion due to the impact of the Yanshan−Himalayan movement. 26 Consequently, trap configurations and fault− fracture systems have undergone varying degrees of alteration.…”

“…25 And the thick mudstone overlying the Suining Formation is used as the effective seals of the region. 28 Combined with the distribution of high-quality reservoirs within the Shaximiao Formation, 60−62 the recommended exploration prospects in the study region are situated primarily in the central and southern regions (Figure 16).…”

“…It is believed that it is characterized by a coal-type gas. There is a consensus among many scholars that the primary gas source originates from the underlying source rocks within the Xujiahe 5 Formation, − but some scholars believe that the Xujiahe 3 Formation and the Lower Jurassic source rocks have a certain contribution. , The main reason for the controversy is that the parameters used for the gas–source comparison are different. As the key to the accumulation of tight sandstone gas, research on the primary source rocks is lagging, − and there is a lack of research on the hydrocarbon generation and expulsion histories of source rocks, which restricts the study of tight sandstone gas reservoir dynamics in this area, resulting in a low exploration success rate.…”