Gas and Condensate Composition in the Deep Tuscaloosa Trend, Southern Louisiana--Influence of Oil and Wet Gas Cracking

George E. Claypool, Melodye A. Rooney, April K. Vuletich, Laurel C. Babcock, Jennifer J. Reber, and Marwin K. Kemp

Natural gas and condensate samples from 34 wells in six fields producing from deep Tuscaloosa sandstones show regular changes in chemical and isotopic composition with increasing depth of burial. A gas-condensate system at 5.2 km (17,000 ft) changes to dry gas at 6.1 km (20,500 ft). Carbon isotopic compositions of ethane and propane become heavier (^dgr¹³C₂ increases from -31 ;to -23 permil; ^dgr¹³C₃ increases from -29 to -21 permil), while methane becomes lighter (^dgr¹³C₁ decreases from -38 to -42 per mil). Depletion of condensate liquids relative to gas over this same depth interval (condensate/gas ratios decrease from 120 to 1 bbl/mmcf) is accompanied by systematic molecular and isotopic changes in the esidual liquids. Higher molecular-weight (>C₂₀) hydrocarbons are progressively depleted, and isoprenoids are lost relative to adjacent normal alkanes. The liquids shift to heavier ^dgr¹³C values (from -27 to -23 permil). These changes are believed to be caused by thermal cracking and progressive conversion of oil and wet gas hydrocarbons to dry gas in Tuscaloosa reservoirs at temperatures of 165 to 195°C.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California