GEOLOGIC CHARACTERISTICS OF THE LESHAN-LONGNVSI PALEO-HIGH, SICHUAN BASIN AND THEIR BEARING ON HYDROCARBON ACCUMULATION

The Gaoshiti-Longwangmiao Gas Field has been an important gas exploration target of Sinian-Cambrian in the western and central Sichuan Basin where gas discoveries have been successively obtained in recent years, including several major ones, such as the Lower Cambrian Longwangmiao gas field. In order to summarize the exploration and research progress in the Sinian-Cambrian in the Sichuan Basin, we made a new round of thorough review of previous studies, and concluded that:(1)The region had been under regional tilting during Late Sinian and the Leshan-Longnvsi Paleo-high was almost in shape, and the region had suffered from syndepositional uplifting during Cambrian and Ordovician.(2)There are two types of gas-bearing reservoirs in the gas field, i.e. fracture-cave type carbonate in the Sinian Dengying Formation and pore type dolomite in the Cambrian Longwangmiao Formation. The penecontemporaneous overlap of dolomitization and karst erosion made available of high porosity and permeability reservoirs, 3%-4% of porosity and (0.0054-9.03)×10^-3μm² of permeability for the Sinian Dengying Formation, and 4%-5% of porosity and (0.0005-78.5)×10^-3μm² of permeability for the Cambrian Longwangmiao Formation.(3)There are multiple layers of source rocks, which could be grouped into two types, in the Sinian-Cambrian of the Sichuan Basin, they are all of sapropel type, favorable for oil generation. There are three sets of high quality reservoirs in the basin with great thickness and extensive distribution.(4)The natural gas in the Sinian and Cambrian is typical dry gas of organic origin in general, except that in the Dengying Formation which was from mixed sources. The natural gas is dominated by crude oil cracking gas in the Cambrian-Ordovician of the Sichuan Basin.(5)The reservoir extensively contact with the source rock. The key factor to cause the great amount of gas accumulation is the contemporaneous development of the Paleo-high with the source rock evolution. Structural, lithological and compound traps developed on a large nose-like uplift provide favorable space for hydrocarbon accumulation.