Origin, Distribution, and Influential Factors of Organic Acids in Deep and Ultradeep Clastic Reservoirs within the Fukang Sag of the Junggar Basin

In deep and ultradeep clastic reservoirs, secondary porosity functions as the primary space for hydrocarbon storage, intricately associated with the dissolution processes of water-soluble organic acids (WSOAs). However, conventional theories concerning secondary porosity predominantly emphasize medium-depth or shallow reservoirs, lacking a thorough investigation into how WSOA-driven mechanisms affect deeper strata formations. To bridge this gap, our research involved selecting 36 samples from Mesozoic Permian clastic rock formations situated in western China's Fukang Sag within the Junggar Basin region. We performed comprehensive analyses utilizing the Soxhlet extraction method combined with qualitative and quantitative assessments via 940 ion chromatography (Metrohm AG). These findings were integrated with oilfield production data to investigate the sources, composition, distribution characteristics, and influencing factors associated with organic acids in deep and ultradeep clastic reservoirs. Our investigation revealed that WSOAs persist even within ultradeep reservoirs; increased buried depths initially lead to a rise in WSOA concentrations followed by a subsequent decline. Similarly, effective porosities closely mirrored these trends alongside variations observed across WSOA concentrations while permeability remained consistently low yet stable throughout these transitions. This indicated significant involvement of WSOAs during dissolution processes contributing to the creation and maintenance of secondary pore spaces. Furthermore, notable positive correlations have emerged establishing a direct relationship between WSOA generation concentrations and corresponding shifts in formation pressures and temperatures. In deep and ultradeep reservoirs, the concentration of organic acids exhibits an initial increase followed by a subsequent decrease in response to escalating formation temperature and pressure. These findings underscore the critical roles played by key influential factors associated with WSOAs in these geological settings.