Petroleum Geochemistry of Devonian Rocks and Produced Oil and Natural Gas in the Caseman - Gross Unit #1 Well, Bradford County, Pennsylvania

Abstract

Predicting the thermal limits of crude oil stability in the earth's crust is a critical component of unconventional resource plays. Recent literature indicates that oil is stable in the subsurface up to temperatures of approximately 200°C. This observation has several implications for assessing the relative contributions of kerogen, bitumen, oil, and light hydrocarbon cracking to natural gases retained in shale-gas reservoirs. The thermal maturity of organic matter in source rocks influences the API gravity and the GOR/COR of hydrocarbon liquids produced from unconventional mudrock and tight-sandstone reservoirs. If the temperature history of an oil accumulation falls within the necessary range, secondary cracking of the oil will proceed.

The cracking of autochthonous post mature kerogen and residual oil was the source of thermogenic gas produced from the Marcellus Formation in Bradford County, Pennsylvania. The Marcellus was also the source of supermature light oil (API = 45.82 heptane ratio = 38.70) produced from shallow Upper Devonian tight sandstone in the Caseman-Gross Unit #1 well in Springfield Township, Bradford County. Various geochemical screening parameters show the produced liquids were generated in the late oil window at burial temperatures of approximately 130 to 150°C (VRe ~ 1.3 – 1.4%) long before the Marcellus Formation reached a maximum burial temperature of ±200°C and its presently observed level of maturity (VRe = 2.7, TAI = 3.7). Light oil produced from the Lock Haven Formation migrated upwards from the Marcellus Formation and escaped complete thermal destruction.

Stable gas isotope data show that associated gases produced with oil from the Lock Haven Formation in the Caseman-Gross #1 well are a mixture of primary and secondary hydrocarbons. The gases are cumulative, and exhibit a partial isotopic reversal with respect to carbon number. Methane mostly consists of late-mature primary gas co-generated with oil through kerogen cracking at VRe ~ 1.36 to 1.4%. Some methane, however, was generated by early oil cracking at a generation temperature of ~175°C. Ethane and propane are secondary gases generated by oil cracking at higher maturity (VRe ~ 1.8%).

Our data provide important constraints for modeling petroleum generation, migration, and preservation in the Marcellus-Catskill/Lock Haven Formations petroleum system in northeastern Pennsylvania.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016