Comparison of the Origin and Secondary Processes of Natural Gas From the Polish Outer Carpathians and the U.S. Bighorn and Wind River Basins

Abstract

This study compares the geochemistry of natural gases occurring within the Lower Cretaceous-Miocene reservoirs of the Polish Outer Carpathians (POC) with those within the Pennsylvanian-Eocene reservoirs of the Bighorn (BB) and Wind River (WRB) Basins in the U.S. Rocky Mountains. Molecular and stable isotope (δ¹³C in C₁-C₅ hydrocarbons and CO₂, δ²H in CH₄) compositions of a suite of 118 samples are examined to understand the processes of petroleum generation, trapping, and secondary alteration in these three basins. The POC, one of the oldest petroleum producing regions in the world, is a fold-thrust belt of Upper Jurassic-Upper Miocene rocks, up to 6 km in stratigraphic thickness. The Oligocene Menilite Formation is the main source rock (petroleum system) with lesser contributions of petroleum sourced from the Lower Cretaceous Spas and Verovice shales, Paleocene Istebna shales and Oligocene-Lower Miocene Krosno beds. The U.S. Bighorn and Wind River Basins were formed during the Late Cretaceous to Eocene Laramide orogeny with most of the oil and gas production from reservoirs of Pennsylvanian, Permian, and Cretaceous age. The main source rocks (petroleum systems) are the Permian Phosphoria Formation, Lower Cretaceous Thermopolis Shale, and the Cretaceous Mowry Shale, with lesser contributions of petroleum sourced from the Upper Cretaceous Frontier Formation, Cody Shale, and Mesaverde Formation, and the Paleocene Waltman Shale Member of the Fort Union Formation.

Results of molecular and isotopic analyses of natural gases accumulated in the POC, BB and WRB reveal that (i) gas dryness and ¹³C-enriched hydrocarbons indicate that most gaseous hydrocarbons are the product of thermogenic processes (POC, BB and WR); (ii) several samples contain ¹³C-depleted CH₄ which suggests a component of microbial CH₄ (POC, BB and WR) that likely was trapped early (in solution or early formed stratigraphic traps) and subsequently mixed with later formed thermogenic gas; (iii) δ¹³C and δ²H values of CH₄ show that POC microbial CH₄ formed by CO₂ reduction; (iv) thermogenic gases were generated at low thermal maturity (oil window) (POC, BB and WR), and occasionally at high thermal maturity (gas window) (WR); (v) ¹³C-enriched values in CO₂ are an indication that some analyzed gases underwent biodegradation (BB and WR, and maybe POC); and (vi) CO₂ was generated, in part, from thermogenic and microbial transformations of organic matter (POC and WR).

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018