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

Guoxiao Zhou,Shengfei Qin,Zheng Zhou,Yu Yang

doi:10.1177/0144598718763902

Guoxiao Zhou, Shengfei Qin + Show 2 more

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https://doi.org/10.1177/0144598718763902

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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 H2S gas. The gases are very dry and the δ13C1 and δ13C2 values display less negative values with an average of −29.2 and −25.0‰, 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 H2S. 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 despite multiple tectonic events in the past. The reservoirs had been in a relatively stable tectonic condition with excellent seals by cap rocks during the gas accumulation period.

Highlights

The Longgang gas field is located on the west side of the Kaijiang-Liangping trough in the northern part of Sichuan Basin, China (Figure 1)
Dominant reef gas reservoirs have been found in the Changxing (P3ch) and Feixianguan (T1f) formations
From top to bottom, the Longgang gas field has a well-developed lacustrine facies source rock in the Lower Jurassic, coal measures acting as terrestrial source rocks in the Xujiahe Formation in the Upper Triassic and coal measure source rocks of marinecontinental transition facies in the Upper Permian Longtan

Summary

Introduction

The Longgang gas field is located on the west side of the Kaijiang-Liangping trough in the northern part of Sichuan Basin, China (Figure 1). The main body of the gas field lies in margin belt of a carbonate platform. It is the largest reef gas field recently discovered in Central Sichuan Basin. The gas field contains the Triassic Xujiahe Formation (T3x) gas reservoir and oil and gas accumulation in the Jurassic (J) formation. From top to bottom, the Longgang gas field has a well-developed lacustrine facies source rock in the Lower Jurassic, coal measures acting as terrestrial source rocks in the Xujiahe Formation in the Upper Triassic and coal measure source rocks of marinecontinental transition facies in the Upper Permian Longtan.

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