Abstract
The geochemical feature and evolutionary history of hydrocarbons from the deep Cretaceous Bashijiqike (K1bs) Formation tight sandstone reservoir in the Dabei Gas Field, Kuqa Depression were investigated using gas chromatography, gas chromatography–mass spectrometry, inclusions petrography and micro-thermometry, laser Raman spectroscopy, and quantitative grain fluorescence. The result indicates that natural gases from the deep sandstone reservoir are mainly composed of alkanes and belong to dry gases, of which methane accounts for 94.30–97.20% (avg. 95.64%), and ethane is 1.23–2.45% (avg. 1.95%). The stable carbon isotopic value of methane and ethane is −31.9‰ to −29.3‰ (avg. −30.3‰) and −24.2‰ to −19.4‰ (avg. −21.7‰), respectively, and this reflects the features of high-mature coal-derived gases. In addition, natural gases in the Dabei Gas Field have characteristics of coal-derived gases which were sourced from Jurassic coal measures. Oils in the Dabei Gas Field predominately originated from Triassic Huangshanjie (T3h) Formation mudstones with some contributions from Jurassic coaly rocks. Petrological and micro-thermometry results of fluid inclusions suggest that the K1bs Formation tight sandstone reservoirs have experienced two phases of hydrocarbons charge histories, namely “early oil and later gas.” The quantitative grain fluorescence analysis indicated that sandstone samples with quantitative grain fluorescence index value >5 and quantitative grain fluorescence-extraction intensity >40 pc in Wells DB101 and DB2 can be used as indicators for the paleo oil layers or the migration channels of later charged natural gas. The aforementioned analyses and burial and thermal histories of K1bs sandstone reservoir demonstrated that oil charged at 10 Ma and natural gas charged at approximately 3 Ma in the study area. Furthermore, paleo-tectonic evolution enabled source rocks to mature and expel hydrocarbons, and the structurally related faults and traps provided pathways and places for hydrocarbon migration and accumulation.
Highlights
The Kuqa Depression, which is located in the north part of the Tarim Basin, north western China, is an area with hydrocarbon-rich resources and broad prospects
The latest explorations indicate that the ultra-deep layer of the Kuqa Depression shows the potential of trillion cubic meters of natural gas, which is one of the most realistic areas for increasing reserves and production in the Tarim Basin (Yu et al, 2019)
This study aims at to (1) investigate the geochemical features and determine the origins of hydrocarbon from deep tight sandstone reservoir in the Dabei Gas Field; (2) provide an approach that analyzes the period and timing of oil and gas charge using petrological characteristics, laser Raman spectroscopy, Th, the quantitative grain fluorescence (QGF) technique, and burial–thermal history; and (3) examine the charge history of hydrocarbon by incorporating the tectonic evolution
Summary
The Kuqa Depression, which is located in the north part of the Tarim Basin, north western China, is an area with hydrocarbon-rich resources and broad prospects. This study aims at to (1) investigate the geochemical features and determine the origins of hydrocarbon from deep tight sandstone reservoir in the Dabei Gas Field; (2) provide an approach that analyzes the period and timing of oil and gas charge using petrological characteristics, laser Raman spectroscopy, Th, the QGF technique, and burial–thermal history; and (3) examine the charge history of hydrocarbon by incorporating the tectonic evolution. The seven sub-structural units are as follows: Yiqikelike Thrust Belt, Kelasu Thrust Belt, Qiulitage Thrust Belt, Northern Moncline Belt, Wushi Sag, Baicheng Sag, and Yangxia Sage (Figure 1(a)) The former three thrust belts were generated during the geological age of 23 À 12 Ma and are the favorable structural units for gas and oil accumulations; the main source rocks are Jurassic coal measures and Triassic lacustrine mudstones (Jia and Li, 2008). This method used short UV excitation wavelengths and recorded continuous spectrums in the range of 300–600 nm
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