Tarim Basin Kongquehe Slope Oil And Gas Accumulation In The Mechanism

Kongquehe Slope was situated in the Northwest of Tarim Basin with an area of 29510km². It is one of the districts which rare study and exploration have been carried out. Themain source rock and exploration potential remain controversial in this area. The aims of thisstudy were to further our understanding of oil and gas accumulation potential. A variety ofinvestigations such as source rock evaluation, oil-source rock correlation, oil and gasaccumulation mechanism and modeling, and reconstruct the process of oil and gasaccumulation were applied in present study. Source rock potentials were evaluated with organic matter abundance, type and maturityfor four possible source rock suites i.e., the Cambrian - Lower Ordovician, Middle and UpperOrdovician, Triassic and Jurassic. The most important source rocks are dark mudstones andcarbonates from the Cambrian- Lower Ordovician, which are organic rich and highly maturedwith type I-II kerogen. Marl and mudstone in the Middle and Upper Ordovician have thesame organic matter type as lower counterpart, moderate organic matter richness and alsohigh maturity, but their contributions in study area are limited due to less thickness. Coal,carboniferous mudstone and dark mudstone in the Jurassic contain type Ⅲ kerogen, but theircontributions are also limited due to low maturity. The main petroleum products in the study area are gas, condensate and heavy oil. Thenatural gases are wet and rich in nitrogen. The carbon isotopic composition of methane andethane are less than -30‰. Terpane distributions of highly matured condensates arecharacterized by high abundance of gammacerane, tricyclic terpanes with the highest peak atC₂₃, low peak at C19, similar abundance of C₂₆ tricyclic as C24 pentacyclic terpanes, equivalentTs and Tm concentration in, while sterane distributions are rich in C₂₇ and C₂₈ sterane, 4α ,23,24-C₃₀ dinosteranes and 4 α ,23,24-C₂₉ triaromatic dinosteranes. The terpanedistributions of heavy oil are also rich in gammacerane and sterane distributions are rich indinosteranes and 4-methyl steranes, triaromatic dinosteranes and 4-methyl triaromaticdinosteranes. These characteristics are well correlated to the Cambrian - Lower Ordoviciansource rocks rather than Jurassic. Natural gases and condensates are mainly derived from oil cracking at high temperaturebased on their compositional features, isotopic values, diamondoid hydrocarbonconcentrations and gas to oil ratios. Oil and gas accumulation time could be inferred fromburial history and fluid inclusion analysis. There are two charges of oil and as occurred inKongquehe Slope, one is in the Hycernian period or late Caledonian period, which wascorresponding to heavy oil or bitumen accumulations in the Cambrian to Silurian reservoirs,the other one is in the Himalayas period which formed natural gas and little condensateresided in Silurian to Jurassic reservoirs. The oil-cracking model provides a guide to exploration in Kongquehe Slope. It is not anoil and gas dead area any more. Early charged oils may be preserved in inherited structuralhigh as normal oils but most possibly these oils are cracked into natural gases which areaccumulated in the Mesozoic and Cenozoic structures whilst these residual oils aretransformed into heavy oil even pyrobitumen.