Characteristics and main controlling factors of Triassic ultra-deep clastic rock reservoirs in Shawan Sag， Junggar Basin： A case study of Karamay Formation in Well Zheng10 area

Abstract

Due to large burial depths， ultra-deep clastic rock reservoirs have poor physical properties and low oil and gas productivity. However， the newly drilled Well Zheng10 in Shawan Sag， Sinopec exploration area， has encountered high-quality thick reservoirs in the Triassic Karamay Formation at a buried depth of 6 700 m， and conventional tests have obtained high industrial oil and gas flow. To reveal the characteristics of this set of reservoirs and favorable main controlling factors， clarify the direction of ultra-deep clastic rock exploration， and reduce the risk of further oil and gas exploration in this area， this paper comprehensively analyzed the data of core， well logging， cast thin sections， physical properties， scanning electron microscopy， and diagenetic evolution of the reservoirs， and discussed the petrological， physical， and pore characteristics of reservoirs in the Triassic Karamay Formation in this area. The results show that the ultra-deep clastic rocks of the Triassic Karamay Formation in Well Zheng10 area are deposited in the front of the braided river delta. The lithology is dominated by glutenites， gravel-bearing fine sandstones， and fine sandstones， and the rock types are mainly feldspar lithic sandstones with low composition maturity. The remaining primary pores and solution pores dominate the reservoir space. The average porosity of the reservoirs is 9.1%， and the average permeability is 2.85 mD. The reservoir belongs to the ultra-low porosity and ultra-low permeability type and low porosity and low permeability type as a whole. Favorable sedimentary facies zones， constructive diagenesis， and abnormal high-pressure control the development and distribution of favorable reservoirs in this area. Among them， sedimentary microfacies are the basis of favorable reservoir development. The fine sandstones deposited in the underwater distributary channel at the front of the braided river delta are better sorted and rounded. The contents of the muddy matrix are low， and the physical properties of the reservoir are better， which are the dominant phase zones for favorable reservoir development. The sodium feldspar cements generated in the early diagenetic process are dissolved by acids in the later period， which increases the dissolution pores and is an essential constructive diagenesis. The abnormal high-pressures formed by continuous oil and gas charging form early and develop continuously for a long time， playing an important construction role. The research results have a guiding role for the subsequent ultra-deep oil and gas exploration in this area.

Keywords

• ultra-deep
• clastic rock
• reservoir characteristics
• main controlling factors
• karamay formation
• shawan sag