Accumulation Mechanism And Model Of Natural Gas In The Leikoupo Formation,Western Depression,Sichuan Basin

The Western Depression of Sichuan Basin is located in the piedmont of Longmen Mountain and Micang Mountain in the West and North of the Sichuan Basin,where the Zhongba gas field with a carbonate reservoir of the Middle Triassic Leikoupo Formation（T₂l）was discovered in 1971.However,the genetic types and sources of the gas have been controversial,as it is difficult to determine whether it is originated from the carbonate source rocks of the T₂l itself or from the deep Permian or Cambrian source rocks.This dispute has affected the evaluation of natural gas resource potential and the exploration process of the T₂l in this area.A series of subsequent drilling failures indicate high risks of the T₂l exploration,whereas Pengzhou,Majing,and Xinchang large and medium-sized gas fields have been discovered since 2010.Therefore,the genetic types and sources of T₂l natural gas,the contribution of T₂l carbonate source rocks to hydrocarbon accumulation,and the resource potential of T₂l become urgent problems to be solved in future exploration and research.Based on the geological,well logging,and seismic data from the boreholes,this study mainly applies sedimentology,petrology,and organic geochemistry methods,combined with petrophysical simulation experiments,to carry out petrological and organic geochemical analysis tests as well as petrophysical experiments.Two core scientific questions,‘The genesis types and sources of natural gas in the T₂l carbonate reservoirs and the contribution of carbonate source rocks with low organic matter abundance（TOC）of the T₂l’,were investigated through:（1）Geochemical characteristics of natural gas and gas-source correlation,（2）The hydrocarbon source rock depositional environment,lithological facies assemblage,distribution and hydrocarbon generation intensity of the T₂l,（3）Fracture mechanism and hydrocarbon expulsion mechanism of the T₂l carbonate source rocks in the piedmont fold-thrust belt in the estern Sichuan Basin,（4）Comprehensive analysis of natural gas accumulation patterns and their main controlling factors.The major findings of this study are as follows:（1）Natural gas in the T₂l is dominated by oil-type gas generated from hydrocarbon source rock pyrolysis,and gas from the Pengzhou,Xinchang,Majing and Yuanba gas fields is mainly derived from low TOC carbonate source rocks of the T₂l itself;（2）The carbonate source rocks interbedded with evaporites in the T₂l have a cumulative thickness between 450 and 950m,with a major TOC distributed between0.1 and 0.4%,gas-prone with a cumulative gas generation intensity of about20×10⁸m³/km²;（3）Gas accumulation in the T₂l is controlled by the Late Indosinian to Himalayan tectonic movement that occurred in the study area,when the low-TOC carbonate source rocks of the T₂l reached high-mature gas window（including the cracking of liquid hydrocarbons trapped in the source rocks to form gas）,and the tectonic movement led to the large-scale fracturing of the gypsum-bearing carbonate source rock successions on the one hand,thereby facilitating the efficient hydrocarbon expulsion and secondary migration from the source rocks,and the formation of anticlinal structural traps that matched the gas migrations in time and space on the other hand,leading to the accumulation of gas directly into the trap to form a gas reservoir.