Characteristics and formation mechanism of mesogenetic dissolution: A case study of Ordovician carbonate in the western slope of the Shulu Sag, Jizhong Depression, Bohai Bay Basin

Abstract Mesogenetic dissolution has the potential to enhance the quality of carbonate reservoirs, and thus it is critical to identify the source of corrosive fluids and establish the mechanism that leads to mesogenetic dissolution. An integrated study using cores, thin sections, seismic profiles, and geochemistry was performed to investigate the characteristics, discuss the possible corrosive fluids, and establish the formation mechanism of mesogenetic dissolution in Ordovician carbonates in the western slope of the Shulu Sag, Bohai Bay Basin. The Shulu Sag underwent a four-stage growth history, and the carbonate rocks in the study area experienced three episodes of diagenetic alteration (i.e., syngenetic, telogenetic, and mesogenetic). The micro- and macroscopic occurrences of mesogenetic dissolution were ubiquitous, including, for example, stylolites, solution seams, tiny pores, laminar dissolution pores, dissolution pores and vugs associated with stylolites, dissolution enlarged fractures, erosion of late minerals (e.g., saddle dolomite), and related hydrocarbon inclusions. The partition patterns of trace elements normalized to Upper Crust Continent (UCC) and rare earth elements (REEs) normalized to Post-Archean Australian Shale (PAAS) were consistent between eroded and uneroded samples. Compared with uneroded samples, eroded samples had higher contents of some trace elements (e.g., Co, Cr, Cu, Ni, V, and Zn) and REEs, and relatively higher average δ13C values and 87Sr/86Sr ratios. These characteristics indicate that the most likely corrosive fluids causing dissolution were organic acids in hydrocarbon-bearing fluids, which may have been derived from the maturation of organic matter in marlstone in the lower part of the Sha3 member (Es3l). The combination of the tilted basin structure and presence of good conduits (i.e., unconformity, stylolites, and fractures) promoted the migration of corrosive fluids and the occurrence of mesogenetic dissolution in Ordovician reservoirs from the Neogene-Quaternary period to the present. A geologic model for the formation mechanism of mesogenetic dissolution has been proposed, which provides new insight for future hydrocarbon exploration.