--> Development, Calibration, and Application of C7 Source, Maturity, and Transformation Parameters

AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis

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Development, Calibration, and Application of C7 Source, Maturity, and Transformation Parameters


Kerogen contains aromatic rings, saturate rings, and branched saturate moieties that structurally are similar to gasoline‐range aromatic compounds (e.g., benzene; toluene), cycloalkanes (e.g., DMCPs; MCH), and branched saturate compounds (e.g., 2‐MH; DMPs). It therefore seems reasonable to conclude that light HC compounds form during the thermal decomposition of kerogen. During the 1980s, Frank Mango (a Shell Oil research scientist trained in catalytic chemistry) proposed a radical alternative: i.e., light HC compounds form during a catalytic process operating at steady state that causes the end units of straight‐chain paraffins to isomerize. The structure of kerogen near sites of active catalysis determines if that process leads to the formation of six‐ membered ring compounds, five‐membered ring compounds, or mono‐ or polybranched alkanes (via ring opening of unstable cyclopropane intermediates) before they are cracked from kerogen. Mango (1986) showed that this kinetic model explains a remarkable invariance in the abundance of four isoheptanes: i.e., 2‐MH, 3‐MH, 2,3‐DMP, and 2,4‐DMP. It also explains a more unusual invariance among several C7 compounds: i.e., 2‐MH, 3‐MH, ΣDMPs, ethyl‐pentane, 1‐DMCP, and 1,3‐DMCPcis+trans. Several C7source parameters (i.e., the X1 and X2 selectivity ratios, which describe isomerization pathways) and a robust C7 thermal maturity parameter (i.e., the 2,4‐DMP/2,3‐DMP ratio) subsequently were identified using this kinetic scheme. Several Shell research geochemists subsequently collaborated with Frank Mango to validate and calibrate this kinetic model. Ray Levey used extracts obtained from mature source‐rock samples to calibrate the C7 tempe‐ rature scale. My principal contribution was to determine how biodegradation and water‐washing influence the values of C7 source and maturity parameters – and then collaborate with Henry Halpern to identify novel gasoline‐range parameters that can be used to characterize biodegraded and water‐washed oils. Later, while I was assigned as the geochemist supporting Shell E&P projects in the deepwater GOM during the early 1990s, I helped determine how the loss of volatile HC compounds from SWCs and core plugs influences C7 source and maturity parameters. That insight now is very valuable to help characterize oil and condensate samples extracted from core plugs selected in quasi‐conventional and unconventional reservoirs (which can be used to reduce uncertainty about which reservoirs are flowing after a horizontal well has been hydraulically fractured). In my poster, I will explain in more detail Mango’s C7 kinetic model and the derivation of C7 source, maturity, and transformation parameters. Then I will discuss examples of how I have used them to: • Determine that Lower Monterey calcareous‐phosphatic shale and Upper Monterey siliceous shale source‐ rock beds generated different kinds of crude oil in the Santa Maria Basin in California • Illustrate the relationship between unaltered and biodegraded Monterey oils that were produced from sandstone reservoirs on the southwestern margin of the Los Angeles Basin in California • Determine the temperature at which oil samples extracted from quasi‐conventional, mudrock, and marl reservoirs were generated in the Midland Basin and on the San Marcos Arch in south Texas These results demonstrate the value of applying C7 source, maturity, and transformation parameters to characterize oil samples produced from conventional reservoirs and unconventional reservoirs.