Process and mechanism for oil and gas accumulation, adjustment and reconstruction in Puguang Gas Field, Northeast Sichuan Basin, China

Abstract

With the discoveries of a series of large gas fields in the northeast of Sichuan Basin, such as Puguang and Longgang gas fields, the formation mechanism of the gas reservoir containing high H 2 S in the ancient marine carbonate formation in superposition-basin becomes a hot topic in the field of petroleum geology. Based on the structure inversion, numerical simulation, and geochemical research, we show at least two intervals of fluid transfer in Puguang paleo-oil reservoir, one in the forepart of late Indo-Chinese epoch to early Yanshan epoch and the other in the metaphase of early Yanshan epoch. Oil and gas accumulation occurred at Puguang structure through Puguang-Dongyuezhai faults and dolomite beds in reef and shoal facies in Changxing Formation (P 2 ch) -Feixianguan Formation (T 1 f) in the northwest and southwest directions along three main migration pathways, to form Puguang paleo-oil reservoir. Since crude oil is pyrolysised in the early stage of middle Yanshan epoch, Puguang gas reservoir has experienced fluid adjusting process controlled by tectonic movement and geochemical reconstruction process controlled by thermochemical sulfate reduction (TSR). Middle Yanshan epoch is the main period during which the Puguang gas reservoir experienced the geochemical reaction of TSR. On one hand, TSR can recreate the fluid in gas reservoir, which makes the gas drying index larger and carbon isotope heavier. On the other hand, the reciprocity between fluid regarding TSR (hydrocarbon, H 2 S, and water) and reservoir rock induces erosion of the reservoir rocks and anhydrite alteration, which improves reservoir petrophysical properties. Superimposed by later tectonic movement, the fluid in Puguang reservoir has twice experienced adjustment, one in the late Yanshan epoch to the early Himalayan epoch and the other time in late Himalayan epoch, after which Puguang gas reservoir is finally developed. main dominance migration pathways, oil and gas adjustment and reconstruction process, thermochemical sulfate reduction (TSR), Northeast Sichuan BasinPrevious studies show that oil and gas reservoir in marine carbonate formation are usually accumulated within steady tectonic environment and mainly during the late accumulation period [1,2] . For the marine carbonate formation in China, four conditions including multi-accumulation, diagenesis, resconstruction, and destruction by multiple tectonic movements are essential for oil and gas accumulation and reservation. Oil and gas reservoirs that were destroyed by uplift, groundwater motion, and fault activation have been investigated extensively [3][4][5][6] . If the area has no extensive denudation or the area has thick effective caprock, reservoir reconstruction is one of the most critical factors to control the distribution hydro-

Petroleum geology features and research developments of hydrocarbon accumulation in deep petroliferous basins

“…The Hugoton gas field (Bai and Cao 2014) and Mills Ranch in the USA's Anadarko Basin, for example, are pure gas fields. Many deep gas reservoirs in China's petroliferous basins also originated from earlier oil or oil-gas reservoirs that were cracked into gas under high temperatures, as exemplified by the large Puguang carbonate gas field (Du et al 2009), the large Kela-2 gas field (Jia et al 2002), and the Hetian gas field (Wang et al 2000). Huge liquid or condensate oil reservoirs have also been discovered in a number of deep basins such as the USA's Rocky Mountain Basin, where gas wells make up only 34 % of the deep exploratory wells while most are oil wells and no pyrolysis has taken place deeper than 6,000 m. This is often because the low subsurface temperature or high pressure of the formation had prevented the crude oil from coming to its threshold pyrolysis temperature (Svetlakova 1987).…”