Carbon Isotopic Characteristics of Jurassic Alkane Gases in the Sichuan Basin, China

Abstract

There are plentiful natural gas resources in Jurassic in the Sichuan basin, where a lot of gas fields (reservoirs) have been discovered, and the natural gas is mainly alkane gas. Comprehensive study on carbon isotopic characteristics indicated that Jurassic alkane gas in Sichuan Basin can be divided into two types, coal-formed gas and oil-type gas. As the majority of the natural gas, coalformed gas distributed in Central and Northwest gas districts and was mainly from the underlying coal-measure hydrocarbon source rocks in Xujiahe Formation. The carbon isotopic series were mainly normal, while a minority were reversed. The reversal ones in Northwest gas district were caused by mixing of gases from the same source rocks with different maturity, while those in Central gas district were caused by mixing of oil-type and coal-formed gases. Oil-type gas was only discovered in Bajiaochang gas field in Central oil and gas district, and derived from self-generation and self-accumulation in Lower Jurassic, with normal carbon isotopic series and without any reversal. Comprehensive analysis suggested that the carbon isotopic characteristics of Jurassic alkane gas were different according to different regions and layers. The δ 13 C 1 and δ 13C 2 values of Jurassic alkane gas in central area were significantly lower than those in northwestern area for the influence by oil-type gas generated from Lower Jurassic Ziliujing Formation.

Geochemical Characteristics and Genetic Types of Natural Gas in the Xinchang Gas Field, Sichuan Basin, SW China

“…The coal-type gases reservoired in T 3 x and the Jurassic strata in the Xinchang gas field have been considered to be derived from coal-measure source rocks in T 3 x (Wu et al, 2010;Dai et al, 2012). The T 3 x source rocks were mainly deposited in terrigenous environment except for the T 3 x 1 in marine-terrigenous facies (Zhu Rukai et al, 2009).…”

“…The natural gas reservoired in the Jurassic stratain the western Sichuan Basin including the Xinchang gas field was believed to be derived from the underlying Xujiahe Formation coal-measure source rocks (Wu et al, 2010;Dai et al, 2012) due to insufficient hydrocarbon potential for the Jurassic strata (Cai Kaiping and Liao Shimeng, 2000), Although the Jurassic-reservoired gases in the Xinchang gas field had been considered to be derived from T 3 x 5 source rocks (Shen Zhongmin et al, 2008), they display similar geochemical characteristics for dryness coefficient, methane carbon and hydrogen isotopic compositions with that of the T 3 x 4 gases. They are also greatly different from the T 3 x 5 gases.…”

“…However, Shen Zhongmin et al (2009) considered that the gas was derived from its own respective set of source rocks, whereas Ye Jun (2001) proposed that the gas was derived from source rocks in the underlying Lower Triassic Ma'antang and Xiaotangzi formations. As for the natural gas reservoired in the Middle-Upper Jurassic strata, since the dark mudstone was hardly developed in western Sichuan Basin, and the Jurassic mudstone had a poor hydrocarbon potential due to the TOC content generally being lower than 0.5% (Cai Kaiping and Liao Shimeng, 2000), the Jurassic gas in western Sichuan Basin including the Xinchang gas field was believed to be derived from the underlying Xujiahe Formation coalmeasure source rocks (Wu et al, 2010;Dai et al, 2012;Wu Xiaoqi et al, 2011). Although Shen Zhongmin et al (2008 proposed that the Jurassic gas in the Xinchang gas field was derived from the 5 th member of Xujiahe Formation source rocks based on the analysis of the rock association characteristics, there was still inadequate geochemical evidence of natural gas.…”

Prospect of Ultra-Deep Petroleum Onshore China

“…The reevaluation according to the new understanding shows that the gas generating amount increased from 1309×10 12 m 3 to 1802×10 12 m 3 in high-evolution coal source rocks in the Ordos, Sichuan, Kuche, South Junggar, Songliao and other areas, the increased amount is 493×10 12 m 3 . In the Xujiahe Formation in Sichuan, for example, the three sets of major source rocks have great potential of hydrocarbon generation, the gas-generating capacity is 10~140×10 8 m 3 /km 2 , the area of gas-generating capacity more than 10×10 8 m 3 /km 2 being 11×10 4 km 2 , and the latest evaluation of the total resources is about 5.5×10 12 m 3 (Wu et al, 2010). The source rocks in the Kuche depression have a big thickness and deep burial, high maturity, strong gas generation, highly abundant resources.…”

Carbon Isotope Evidence for Ordovician Marine Hydrocarbon Source Rocks in Ordos Basin, North China

“…Carbon isotope of marine carbonate can approximately reflect the inorganic carbon isotope of ancient seawater. Productivity and organic carbon burial are two important influencing factors on carbon isotope of marine carbonate (Wu et al, 2010). High productivity and rapid burying of organic carbon often indicates selective absorbing of light carbon isotope ( 12 C) in sedimentary organic matter.…”