Guidelines for Type II-S Kerogens in Basin Modeling: Organic Sulfur Content of Kerogens and Crude Oils

Abstract

Kerogens rich in organically bound sulfur (type II-S or high sulfur) generate oil at lower thermal exposure than more familiar type II kerogens. Initial oils have high sulfur content (3-6 wt. %), low API gravities (10-20[degrees] API), and are expelled at low kerogen maturities (R[sub o] [approximately] 0.3-0.4). Oil quality improves significantly as kerogen maturity increases such that oils from the two kerogen types are virtually indistiguishable when oil gravity exceeds about 35[degrees] API. Different kinetic parameters characterize oil generation from type II and type II-S kerogens. Depending on which kinetic model is selected, results of quantitative basin modeling are very different, especially in timing of oil generation and significant expulsion. Geochemical criteria for selecting between type II and type II-S kerogens for basin modeling are (a) the atomic S[sub org]/C ratio of the source kerogen; (b) the atomic S[sub org]/C ratio of the asphaltene fraction of oils, oil-stained rocks, and tar seeps; and (c) the relationship between %S and API gravity for crude oils. Geological criteria for predicting which kerogen type(s) may be influential in a particular depocenter can result from thoughtful analysis of the depositional environment and lithology of the known or presumed source rock. Dysoxic or anoxicmore » bottom waters beneath highly productive near-surface waters, and environments with limited terrigenous sediment input, are the favored settings for formation of type II-S kerogens. Worldwide, the occurrence of high-sulfur kerogen should be considered wherever potential source rocks are in thick carbonate, marl, diatomaceous, or evaporitic sequences. A global survey suggests that major fraction of the world's crudes originated from high-sulfur kerogens.« less