Basic characteristics and accumulation mechanism of Sinian−Cambrian giant highly mature and oil-cracking gas reservoirs in the Sichuan Basin, SW China

Bing Luo,Yu Yang,Gang Zhou,Shujiao Shan,Maolong Xia,Wenjun Luo

doi:10.1177/0144598717736856

Abstract

Old Mesoproterozoic−Cambrian successions have been regarded as an important frontier field for global oil and gas exploration in the 21st century. This has been confirmed by a recent natural gas exploration breakthrough in the Sinian and Cambrian strata, central Sichuan Uplift, Sichuan Basin of SW China. However, the accumulation mechanism and enrichment rule of these gases have not been well characterized. This was addressed in this work, with aims to provide important guidance for the further exploration while enriching the general studies of the oil and gas geology in the old Mesoproterozoic–Cambrian strata. Results show that the gas field in the study area is featured by old target layers (Sinian–Lower Cambrian), large burial depth (>4500 m), multiple gas-bearing intervals (the second and fourth members of the Sinian Dengying Formation and the Lower Cambrian Longwangmiao Formation), various gas reservoir types (structural type and structural–lithologic type), large scale (giant), and superimposing and ubiquitous distribution. The giant reserves could be attributed to the extensive intercalation of pervasive high quality source rocks and large-scale karst reservoirs, which enables a three-dimensional hydrocarbon migration and accumulation pattern. The origin of natural gas is oil cracking, and the three critical stages of accumulation include the formation of oil reservoirs in Triassic, the cracking of oil in Cretaceous, and the adjustment and reaccumulations in the Paleogene. The main controlling factor of oil and gas enrichment is the inherited development of large-scale stable paleo-uplift, and the high points in the eastern paleo-uplift are the favorable area for natural gas exploration.

Highlights

Mesoproterozoic–Cambrian successions are traditionally not viable oil and natural gas exploration targets due to their long and complex geological history and relatively great burial depth, and discovered petroleum resources there only account for less than 1% of the total geological strata (Klemme and Ulmishek, 1991; Wang and Han, 2011)
With the depletion of hydrocarbon reserves hosted in shallower rocks, Mesoproterozoic–Cambrian units are increasingly being explored for their hydrocarbon potential, resulting in significant discoveries, such as the Khazzan gas field in Oman (Rylance et al, 2011), the breakthrough in Amadeus Basin, Australia (Edgoose, 2012; Swanson-Hysell et al, 2012), and the success in Tarim and Sichuan basins in China (Zheng et al, 2013; Zhu et al, 2012; Zou et al, 2014a)
Some of these reservoirs occur at the boundary between the Cambrian and the Proterozoic, indicating that they may be of general geological significance (Carminati et al, 2010; Du et al, 2012; Liang et al, 2009; Pedersen et al, 2007; Sahu et al, 2013; Zou et al, 2014a)

Summary

Introduction

Mesoproterozoic–Cambrian successions (ca. 16 Ga–485 Ma) are traditionally not viable oil and natural gas exploration targets due to their long and complex geological history and relatively great burial depth, and discovered petroleum resources there only account for less than 1% of the total geological strata (Klemme and Ulmishek, 1991; Wang and Han, 2011). There are multiple unconformity surfaces and a large number of fault systems in the study area, which effectively connects reservoirs with source rocks to form a mesh-like migrating system (Figure 6), which is significantly favorable for the largescale hydrocarbon migration and accumulation in the paleo-uplift.

Results

Conclusion