How organic matter types in source rocks influence reservoir diagenesis: Evidence recorded in carbonate cements

Abstract

Fluids generated from the source rocks containing various kerogen types at different thermal maturity stages control diagenetic processes and reservoir quality in adjacent sandstone reservoirs. This study focuses on the carbonate cements in the sandstones of the Lower Jurassic Yangxia Formation and the Ahe Formation in the Tarim Basin. The δ¹⁸O, δ¹³C, and ⁸⁷Sr/⁸⁶Sr data indicate that low‑temperature ferroan calcite and manganoan calcite—characterized by strongly negative δ¹³C values and enrichment in light rare‑earth elements (LREEs)—record CO₂ released during the thermal degradation of organic matter predominantly composed of Type III kerogen in coal‑bearing source rocks and of Type II kerogen in mudstone source rocks, respectively. High‑temperature ferroan calcite and manganoan calcite, which exhibit similarly strongly negative δ¹³C values and enrichment in middle rare‑earth elements (MREEs), record organic acids and CO₂ produced during the thermal decarboxylation of these same source rocks. The diagenetic fluid evolution sequence comprises early‑stage CO₂ from thermal degradation of both coal‑bearing and mudstone source rocks; mid‑stage organic acids and CO₂ from thermal decarboxylation of coal‑bearing source rocks; and late‑stage organic acids and CO₂ from thermal decarboxylation of mudstone source rocks. Fluids generated during the thermal degradation of mudstone and coal‑bearing source rocks precipitated extensive calcite cements, leading to reservoir densification. Clumped isotope thermometry indicates that the primary generation periods of late‑stage mudstone‑derived fluids coincided with the formation of effective fractures. Feldspar dissolution along these fractures produced an interconnected network of fractures and dissolution pores, significantly enhancing reservoir quality in the Ahe Formation.