INTEGRATED HYDROCARBON CHARGE EVALUATION OF HAYNESVILLE AND BOSSIER GAS SHALE RESERVOIRS, NORTHERN LOUISIANA

Gas-bearing shales in the Upper Jurassic Haynesville Formation and the overlying Bossier Formation in northern Louisiana are a new energy source. These unconventional reservoirs contain gas generated by indigenous kerogen that was retained in intergranular matrix porosity and kerogen nanoporosity. We applied integrated HC charge evaluation methods to help identify shale intervals with the best commercial potential. Our study involves three complementary elements: (1) characterizing the source rock (SR) potential of Bossier and Haynesville shales; (2) analyzing the composition of gas samples obtained from different shale intervals; and (3) modeling oil and gas generation by Bossier and Haynesville SRs. We identified two good SR intervals: Upper Haynesville/Basal Bossier shales contain ~1.5-5.0 wt% total organic C (TOC); a second SR interval in the Middle Bossier Formation contains ~1.5-3.0 wt% TOC. Upper and Lower Bossier shales generally are much leaner (<1.5 wt% TOC). Bossier and Haynesville shales have reached such a high level of thermal maturity (VR ~ 2.0-2.8) it is difficult to determine what kind of kerogen they originally contained. But the presence of pyrobitumen in Haynesville SRs and the abundance of gas-prone and inert kerogen in lean Bossier shales indicate only the two good SR intervals contained oil-prone kerogen. Basin modeling indicates the volume of gas generated by Haynesville and Middle Bossier SRs exceeded their intrinsic storage capacity, while lean gas-prone Bossier shales generated less gas than they could retain. Excess gas expelled by good SRs probably migrated through the leaner shales via fractures and faults. The molecular and C isotopic composition of gas samples produced from Haynesville and Middle Bossier shales support these intepretations: e.g., C₁/C₂ ratios (~1000-2100) indicate the gas was generated at very high maturity. Systematic changes in gas composition across the study area indicate that the maturity of kerogen in each good SR largely controls gas wetness and the C isotopic composition of HC gases: i.e., the reservoirs are compartmentalized vertically and laterally. Mud gas samples from lean Bossier shales are much wetter than mud gas samples obtained from good SR intervals, suggesting Upper and Lower Bossier shales do not contribute significantly to production.