Study on the origin and hydrocarbon generation potential of lacustrine organic-rich dolomite affected by volcanism: A case study of Lucaogou Formation in the Malang Sag, Santanghu Basin, Western China

Abstract

Lacustrine organic-rich laminated dolomites are widespread in the Permian Lucaogou Formation (P2l) of the Santanghu Basin. However, the origin of the dolomites remains controversial due to the influence of volcanism and complex sedimentary environment. Therefore, a composite analysis of mineralogical and geochemical is utilized to study the origin and hydrocarbon generation potential of the P2l dolomite. The results showed that the P2l dolomite was the product of penecontemporaneous dolomitization, burial dolomitization, and local hydrothermal dolomitization. The evidence was as follows: (1) Under microscopy, a large number of dolomites had obvious trigonal rhomboid and zonal structure, indicating the existence of metasomatism. Besides, the P2l was characterized by an arid climate, suboxic-anoxic condition, and high salinity of the lake, with some evaporite minerals and low-order degree of the dolomite (0.4–0.6), which suggested that the dolomite was formed by penecontemporaneous rapid crystallization; (2) Part of the dolomites were characterized by a high content of Fe2+ and Mn2+, a moderately positive excursion of δ13CV-PDB values (2–12‰, avg. 7.4‰), a negative excursion of δ18OV-PDB values (−20 to −2.3‰, avg. −10.6‰), a high-order degree (0.7–0.8), a positive anomaly of Eu, and a certain enrichment of light rare earth elements (LREEs), indicating that it was caused by burial dolomitization; (3) Most of the dolomites were ankerite, and the speckled dolomite and veins were mostly developed near the fault zone, with a lower value of δ18OV-PDB (<−8‰), an obvious positive anomaly of Eu (1.01–1.68, avg. 1.28), and an obvious enrichment of LREEs, indicating a higher formation temperature. A large number of algal blooms caused by volcanic ash led to a high abundance of organic matter (OM) in the P2l dolomites, with total organic carbon (TOC) values ranging from 0.8 to 7.3% (avg. 2.7%). The lower limits of the TOC of the P2l dolomite as an effective source rock were determined to be 2.1%. Moreover, the hydrocarbon potential of the P2l dolomite, with abundant oil-prone OM (Type I and II1 kerogen with hydrogen index (HI) values ranging from 142 to 975 mg/g) in the stage of early mature to mature (Peak temperature of pyrolytic hydrocarbon peak (Tmax) and vitrinite reflectance (Ro) values ranging from 430 to 451 °C and 0.3–0.96%, respectively), is considerable. The values of the oil saturation index (OSIS1/TOC × 100) are 6.1–479 mg/g (avg. 59 mg/g), indicating that the P2l dolomite contains producible oil. High contents of brittle minerals and TOC, and traces of clay minerals show that the P2l holds a significant hydrocarbon generation potential.