Geochemical characteristics of natural gases from different petroleum systems in the Longgang gas field, Sichuan Basin, China

Abstract

Located in the Sichuan Basin, China, the Longgang gas field consists of three vertically developed petroleum systems with the Triassic Leikoupo Formation as a dividing interface. There is one marine petroleum system below the interface and one continental petroleum system above it. The marine petroleum system is composed of coal measures, the main source rock in the Longtan Formation, and marine reef reservoirs in the Changxing and Feixianguan formations. The continental petroleum system can also be subdivided into two sets. One is the Xujiahe petroleum system sourced from the Xujiahe coal measures in the Upper Triassic formation. The other is a Jurassic petroleum system that is sourced from Jurassic lacustrine black shales. The gas pools in the marine system contain H 2 S gas. The gases are very dry and the d 13 C 1 and d 13 C 2 values display less negative values with an average of À29.2 and À25.0&, respectively. The gases are humic origin generated at highly to over mature stages from coal measures of the Longtan Formation. The natural gas in the continental petroleum system does not contain H 2 S. The natural gases from the Xujiahe petroleum system are mainly wet gases with a few dry gases, and belong to typical humic type sourced from coal measures of the Xujiahe Formation. All the gases from this Jurassic petroleum system are wet gases and the alkane gases show more negative carbon isotopic values typical of sapropels. These are derived from the lower Jurassic lacustrine black mudstone. The three sets of petroleum systems in the Longgang gas field are vertically well separated. Each system has its own source rock, and there are no gases from other sources

Genetic Types, Distribution Patterns and Enrichment Mechanisms of Helium in China’s Petroliferous Basins

“…It is also related to the water-soluble gas. The natural gas in the Hetianhe gas field mainly comes from the high-over-mature Cambrian marine source rocks in the Southwest Depression to the field (Zhao, 2000;Cai et al, 2002;Qin et al, 2002). Carbon isotopic values of methane in the western part of the field are significantly less negative than those in the gas in the eastern part of the field.…”

“…The continuous decrease in pressure during the migration of the water body caused the continuous release of water-soluble gas to form free natural gas phase, which accumulated along the migration path, forming the Hetianhe gas field in a strip-like reservoir (Qin et al, 2006). The long distance gas migration has caused the difference in the geochemical characteristics of natural gas in the east and west areas of the gas field (Qin et al, 2002). Although research is ongoing on whether there is an ancient granite basement in the Southwest Depression of Tarim Basin, U and Th in the ancient basement strata and Cambrian source rocks have produced considerable amount of helium gas through radioactive decay over a long period of time.…”