A review of carbonates as hydrocarbon source rocks: basic geochemistry and oil–gas generation

Abstract

Carbonates have been known to act as hydrocarbon source rocks, but their basic geochemical and associated hydrocarbon generation characteristics remain not well understood as they occur with argillaceous source rocks in most cases, and the hydrocarbon generation from each rock type is difficult to distinguish, forming one of puzzling issues within the field of petroleum geology and geochemistry. To improve the understanding of this critical issue, this paper reviews recent advances in this field and provides a summary of key areas that can be studied in future. Results show that carbonate source rocks are generally associated with high-salinity environments with low amounts of terrestrial inputs and low dissolved oxygen contents. Petrographically, these source rocks are dark gray or black, fine-grained, stratified, and contain bacterial and algal bioprecursors along with some other impurities. They generally have low organic matter contents, although these can vary significantly in different cases (e.g., the total organic carbon contents of marine and lacustrine carbonate source rocks in China are generally 0.1%-1.0% and 0.4%-4.0%, respectively). These rocks contain type I and type II kerogen, meaning there is a lack of vitrinites. This means that assessment of the maturity of the organic matter in these sediments needs to use non-traditional techniques rather than vitrinite reflectance. In terms of molecular geochemistry, carbonate source rocks have typical characteristics indicative of generally reducing and saline environments and lower organism-dominated bioprecursors of organic matter, e.g., high contents of sulfur compounds, low Pr/Ph ratios, and dominance of n-alkanes. Most of the carbonate source rocks are typically dominated by D-type organic facies in an oxidized shallow water mass, although high-quality source rocks generally contain A-and B-type organic facies in saline lacustrine and marine-reducing environments, respectively. The hydrocarbon generation model for the carbonate source rocks can involve early, middle, and late stages, with a diversity of hydrocarbons within these rocks, which can be aggregated, adsorbed, enclosed within minerals, or present as inclusions. This in turn implies that the large-scale hydrocarbon expulsion from these rocks is reliant on brittle deformation caused by external forces. Finally, a number of aspects of these source rocks remain unclear and need further study, including the effectiveness of carbonates as hydrocarbon source rocks, bioprecursors, and hydrocarbon generation models of carbonate source rock, and the differences between marine and lacustrine carbonate source rocks.

Genesis of diagenetic zeolites and their impact on reservoir formation in the Middle Permian Lower-Wuerhe Formation of the Mahu Sag, Junggar Basin, Northwest China

“…Laumontite dissolution is common in the study area (Figures 3(i) and 8), whereas heulandite dissolution is rare. Organic acids produced during thermal evolution of organic matter may result in laumontite dissolution (Li et al., 2014; Wu et al., 2018, Xia et al., 2019). It is clear that laumontite contents and dissolution in the fan-delta front are related to the reservoir porosity, permeability, and well production according to database of reservoir property, test, and production in the Mahu Sag.…”

Method for identifying effective carbonate source rocks: a case study from Middle–Upper Ordovician in Tarim Basin, China

“…The evaluation method may be more objective, especially for the source rocks at high evolution stage, such as those in the Tarim Basin and Sichuan Basin in China. Currently, lacustrine carbonate source rocks have also been noted during recent years (Liu et al 2019). For example, the carbonate source rocks might have act as important oil contributors in the lacustrine Mahu sag (Cao et al 2019).…”

Formation environments and mechanisms of multistage paleokarst of Ordovician carbonates in Southern North China Basin

“…Carbonate karst is of great importance for carbonate karst reservoirs with high oil and gas reserves, including 52% of the reserves of oil and gas and about 60% of oil and gas outputs in the world 1 . Among the eight major oil and gas basins in the world, except for the Sichuan Basin in China and the East Siberian Basin in Russia, the oil and gas reservoirs in the Tarim Basin, Ordos Basin, Permian Basin, Williston Basin, Michigan Basin, and the Oman Basin, are all related to the Lower Paleozoic Ordovician and/or Cambrian carbonate paleokarst 1 – 3 . Therefore, an in-depth study of the developmental characteristics and formation mechanism of the Lower Paleozoic carbonate paleokarst will provide great insights and guidance for scientific exploration, development, and management of major oil and gas basins in the world.…”

“…In Northern China, oil and gas reserves are mainly concentrated in the Lower Paleozoic Ordovician carbonates in the Tarim Basin, Ordos Basin, and Bohai Bay Basin, where more than 60% of the total oil and gas production in China comes from 1,3 . Many researchers have found that the Ordovician carbonates in Northern China recorded not merely one, but multistage paleokarst, and different regions have rather site-specific formation environments and stages of the Ordovician paleokarst [4][5][6] .…”