Geochemical Characteristics and Origin of the Lower Paleozoic Gas in the Tazhong Uplift, Tarim Basin

Abstract

The Tazhong uplift is located in the central Tarim Basin covering an area of 3×104km2. It has experienced strong multiphase tectonic movements, leading to the formation of the No.1 Fault Belt and the North Slope. The lower Paleozoic carbonate reservoirs of Tazhong uplift is rich in oil and gas for widely developing two sets (ε–O1 and O2+3) of marine source rocks and thick-layer dolomite reservoirs. However, there are different opinions on the origin and genetic types of oil and gas due to the two similar type source rocks and complex tectonic settings. In order to distinguish the origin and genetic types of natural gas, a detailed analysis of geochemical characteristics of 72 natural gas samples and experimental simulations for two sets of marine source rocks in the study area were carried out. The ε-O1 (TOC=3.6%，Ro=0.74%) and O2+3 (TOC=2.1%，Ro=0.72%) source rocks for simulation were selected from 35 fresh outcrops in the Kepin region and drilling cores of the Tazhong uplift. Experiments were conducted in open and closed systems under different temperatures and pressures. After simulation, the generated gas was analyzed by Thermo Fischer Scientfic equipped with an Agilent 6890 N GC. Compared with the simulation results, the δ13C2＜-35.4‰ of natural gas was obtained to divide natural gas derived from ε-O1 source rocks. Through the analysis of composition and carbon isotope of natural gas, simulation of source rocks, and selecting the ln(C1/C2)-ln(C2/C3), C1/C2-C2/C3, i/nC4-i/nC5 plates for marine natural gas, the origin and genetic types of natural gas were analyzed. The results indicate that natural gas of the study area is typical oil-type gas. In the North Slope, the natural gas is oil cracking gas derived from ε-O1 source rocks with relatively high dryness coefficient, high maturity and high content of non-hydrocarbons than the No.1 Fault Belt. While the natural gas in the No.1 Fault Belt is mixture gas of oil cracking gas and kerogen cracking gas derived from ε-O1 and O2+3 source rocks in different thermal evolutional stage. The high content of H2S, CO2 and partial reverse between C2H6 and C3H8 (δ13C2＞δ13C3)in the North Slope is caused by thermal reduction reaction of sulphate and high thermal evolution level.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018