Re-examination of genetic types and origins of natural gases from Dibei bulge, eastern Luliang uplift, Junggar Basin, China

Abstract

The eastern part of Luliang uplift has currently become the main target for gas exploration in the Junggar Basin. By now, several giant–medium gas fields (reservoirs) are discovered in the Dinan bulge. Located north to the Dinan bulge, the Dibei bulge is surrounded by several hydrocarbon generating sags and developed several sets of source rocks, which promises a favorable gas accumulation condition. Based on molecular and stable carbon isotopic compositions of nine gas samples from the Dibei bulge, in cooperation with the burial and hydrocarbon generation history of source rocks, genetic types and origins of natural gases and related secondary alterations were fully discussed in this study. A comparative study was also carried out with the geochemical characteristics of natural gases from the Dinan bulge. Gases from the Dibei bulge have complicated origins which could primarily be divided into three types: (1) highly-mature coal-derived gas generated from the Upper Carboniferous Batamayineishan coal measures; (2) highly-mature oil-type gas generated from the marine source rocks in the Lower Carboniferous Jiangbasitao Formation or Songkaersu Formation; (3) secondary microbial methane generated via the biodegradation of oils. Significant differences have been observed between gases from the Dibei and Dinan bulges in the concentration of non-hydrocarbon gases, carbon isotopes and thermal maturities. Gases from the Dibei bulge are at highly mature stage, which correlates well with the maturities of Carboniferous source rocks in the Ulungu depression. Thus, these gases are most probably derived from the Ulungu Depression.