Abstract:

Leikoupo Formation has become an important target in natural gas exploration of the Sichuan Basin in recent years. Some disputes are remained on both the natural gas source and its generation potential for the Leikoupo Formation. It is indicated that the mean TOC for the argillaceous marl and dolomite arrived at 0.72%, with a comparative high capacity of gas generation. The natural gases are dominated by hydrocarbon component with a low amount of heavy hydrocarbons (round 1%) and have a dryness index of 0.99. Unlike the natural gases found in other marine sequence, the natural gases investigated in this paper contain relatively low concentration of no-hydrocarbons (abundances of CO₂ and N₂ less than 10% and H2S in trace amount (<0.01%)). The methane is isotopically somewhat heavy (-31.5‰ to -36.3‰), whereas the ethane has relatively light values mostly less than -28‰ (-27.7‰ to -36.6‰), with isotopic reversals in partial gas samples, suggesting a post-mature marine gas. Natural gases in Leikoupo Formation are distinguished in isotopic compositions of alkane hydrocarbons from typical terrestrial gas in Xujiahe Formation and marine gas in Feixianguan-Changxing Formation, indicating a different gas source. In conjunction with the character of hydrocarbon source and distribution of source rocks, the close similarity of light hydrocarbons with marls of Leikoupo Formation indicate that the natural gases in Leikoupo Formation are likely from the source rock within this sequence. Through the integrated investigation for the gas accumulation in the Leikoupo Formation, it is found that the source rock occurrence controlled the gas enrichment. Namely, natural gas was only accumulated in “lithologic traps” (constituted by shoal dolomite and/or fractured limestone) which located near the source rock.

Key words: natural gas, source rock, accumulation, Leikoupo Formation, Yuanba area