Source Rock Organofacies, Development and Preservation: The Basis for a Quantitative Description of Source Rock Expulsion Potential, Expelled Fluid Composition, and Properties - in Time and Space

Abstract

The Petroleum System is a machine driven petroleum fluid volume creation in the source bed. Rates of expulsion from Organic Matter (OM) are vanishingly slow and these rates ultimately control all migration rates, determining the progression of the lateral or vertical charge fronts in the basin; and limiting the rates of reservoir charging and leakage. The two factors limiting the mass of petroleum expelled from the OM in the source bed are: 1) its initial expulsion potential (mass and composition); and 2) the cumulative fraction of potential expelled up to its maximum state of maturation. The four most important parameters determining initial expulsion potential are: thickness, organic carbon (Corg), Hydrogen Index (HI) and Organofacies. In its most simple form, composition can be modeled as the proportions of two petroleum carbon (PC) fractions: PC1-5 “gas” and PC6+ “oil”. This mass ratio can then be converted to “oilfield” volumetric equivalents and related to subsurface physical properties such as density and viscosity at the ambient pressure and temperature conditions. HI is the most important in determining composition, since it governs the ratio of reactive to inert carbon in the OM and thereby controls the mass of generated PC6+ (“oil”) that can be expelled from the organic matrix, once its sorption capacity is exceeded. Un-expelled oil is cracked and is added to the PC1-5 mass expelled from the gas-generating kerogen portion of the OM, increasing significantly the expelled gas-oil ratio. Note: HI is largely independent of Organofacies, since oxidation can reduce the preserved HI irrespective of its biological origin. In the classical Tissot scheme, Type III OM with low HI is not necessarily terrigenous in origin! Organofacies control the kinetics of generation of the PC1-5 (“gas”) and PC6+ (“oil”) components and the increasing temperature (from A to F) required to generate PC6+ sufficient to exceed the sorption capacity of the OM. Terrigenous organic matter (Organofacies D to F) has increasingly high oil generation temperatures which increasingly overlap with oil-to-gas cracking; this compounds their tendency to have low HI / low expulsion efficiency, explaining why these Organofacies are associated typically with high GOR through dry gas systems. Thickness and Corg are scalar factors that largely influence gross mass / volume rather than composition. Note: they are often negatively correlated; there are few thick and organic-rich source rocks! However, “starting with the end in mind”, this thorough description of the initial expulsion potential of source rocks remains an often over-looked step in Petroleum Systems Analysis. Terms such as “world class source rock” have been so over-used as to make them little more than an advertising slogan. Many modern basin modeling packages still proliferate the problem by requiring a single number per cell, as input to the kinetic model calculation of generated and expelled masses and volumes – essentially a hang-over from the low computing power of the 1980’s when kinetic modeling originated. Present-day computing power now allows us to model the effects of “real” vertical and lateral changes in source rock Organofacies, richness and quality arising from the conditions under which organic matter was developed and preserved. In turn, this now behooves us to go “back to the source” to revise / develop new workflows in mapping and quantifying the raw material from which the Petroleum System originates. Therefore, to evaluate the initial expulsion potential quantitatively, we introduce a workflow to estimate the Ultimate Expellable Potential (UEP), which represents the cumulative mass of oil and gas that can be expelled upon complete maturation of the source rock. For use in resource estimation, these masses can be converted to surface volumes of oil and gas per unit area (UEO in mmstb/km2 and UEG in bscf/km2 or mmboe/km2, respectively). UEO, UEG and UEP can be mapped across the depositional extent of the source bed, just as a reservoir depositional system can be mapped. [Global examples of such maps are included in an accompanying poster in this session.] Volumetrically significant petroleum systems are supplied by source beds with 10’s of mmboe/km2; with the world’s most prolific source beds reaching 100mmboe/km2 or more. Constructing logs of UEP plotted in the time rather than the depth dimension, it also becomes apparent that there are many global coincidences in the times at which source rock potential “Acmes” were deposited, providing a feedback loop into the causal mechanisms of the originating OM depositional events. The last decade or so of exploitation of organic-rich “shale” reservoirs has created large volumes of detailed geochemical and petrophysical data on source rocks, that were rarely available in the often sparsely-sampled conventional exploration world. Here, it has become easier to see how changes in organic matter properties result in differences in “shale” reservoir fluid properties such as GOR and viscosity and in some cases the storage characteristics of the reservoir rock itself. [This topic will be expanded in a talk by the author in Session 3.]

AAPG Datapages/Search and Discovery Article #90337 ©2019 AAPG Middle East Region GTW, Regional Variations in Charge Systems and the Impact on Hydrocarbon Fluid Properties in Exploration, Dubai, UAE, February 11-13, 2019