ABSTRACT

The Lawton oil field is located on the Wichita Mountain uplift in southwestern Oklahoma. This study focused on addressing three problems related to the Lawton oil field: (1) What is (are) the source(s) of the oils? (2) Have the oils been subjected to alteration since emplacement? (3) What is the filling history of the field? Eighteen oils and one seep sample were collected and characterized by a variety of geochemical techniques including bulk composition, biomarkers, and stable carbon isotopes to address these questions. Source rocks in this region are absent following the Pennsylvanian Wichita orogeny, but biomarkers and stable isotopic data suggest the source of these oils is probably the Woodford Shale. Despite the fact that all of these oils were recovered from reservoirs at depths between 125 and 1000 ft (38 and 305 m), none of them appeared to be extensively biodegraded as manifested by the abundance of the complete range of n-alkanes. However, the presence of 25-norhopanes in all 18 samples suggests that these oils are mixtures of various proportions of degraded and nondegraded (or slightly degraded) oils. The seep sample and some oils have a relatively high abundance of the 17β(H), 21β(H) 22R bishomohopanoic acid (C32 ββ 22R hopanoic acid), relative to the 17β(H), 21β(H), 22R homohopane (C31 ββ 22R hopanoic acid), with the presence of these ββ 22R hopanoic acids suggesting the oils have been degraded aerobically at some point in their history. It was also noted that the concentration of the C31 ββ 22R hopanoic acid is higher in the shallower reservoirs than in the deeper reservoirs, possibly indicating slightly higher levels of degradation in the shallower reservoirs compared to the deeper reservoirs. A hydrocarbon accumulation model has been proposed for this field based on two main reservoir charging periods. The initial charge of crude oil was biodegraded, probably under anaerobic conditions. The second charging period occurred after (or together with) the uplift of the Wichita Mountains and subsidence of the southern part of the Anadarko Basin, leading to deeper burial of the source rock and production of higher-maturity oil, which subsequently filled the reservoirs containing the degraded oil and may still be recharging these shallow reservoirs today.

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