Our organic geochemical study of oils from the northern Qaidam basin defines a family of genetically related oils that contain biomarkers indicative of source rocks deposited in Tertiary hypersaline, anoxic lacustrine settings. Although Cenozoic outcrop samples from northern Qaidam are too organic lean to be of source quality, dark laminated upper Oligocene mudstones containing gypsum crystals and pyrite from the Shi 28 well yield total organic carbon (TOC) and Rock-Eval data indicative of fair to good source rocks. Organic matter is derived from algae and bacteria and there apparently was little contribution from terrestrial material. Biomarker data provide a good correlation between the produced oils and the upper Oligocene Shi 28 core samples. Hydrocarbons derived from these source rocks are contained in upper Oligocene, Miocene, and Pliocene reservoirs. Although eight of the oil samples are from the northwest corner of the basin, one sample in this genetic family of hypersaline oils comes from northeast Qaidam, an area previously believed to only produce oils derived from Jurassic freshwater lacustrine source rocks. This sample thus indicates the presence of an unidentified and undocumented source rock in the northeast part of the basin. Hypersaline oils and the associated source rocks have low biomarker maturity parameters. Thermal modeling indicates that hydrocarbon generation probably occurred in northwestern Qaidam within the last 3 million years.
New bulk and molecular organic geochemical analyses of source rock and oil samples from Mongolia indicate the presence of lacustrine-sourced petroleum systems in this frontier region. Samples of potential source rocks include carbonate, coal, and lacustrine-mudstone lithologies that range from Paleozoic to Mesozoic in age, and represent a variety of tectonic settings and depositional environments. Rock-Eval and total organic carbon data from these samples reflect generally high-quality source rocks, including both oil- and gas-prone kerogen types, mainly in the early stages of generation. Bulk geochemical and biomarker data indicate that Lower Cretaceous lacustrine mudstone found in core from the Zuunbayan field is the most likely source facies for the East Gobi basin of southeastern Mongolia. Oil and selected source rock samples from the Zuunbayan and Tsagan Els fields (both in the East Gobi basin) demonstrate geochemical characteristics consistent with nonmarine source environments and indicate strong evidence for algal input into fresh- to brackish-water source facies, including elevated concentrations of unusual hexacyclic and heptacyclic polyprenoids. Despite similarities between Zuunbayan and Tsagan Els oil samples, biomarker parameters suggest higher algal input in facies sourcing Zuunbayan oil compared to Tsagan Els oil. Tsagan Els oil samples are also generated by distinctly more mature source rocks than oil from the Zuunbayan field, based on sterane and hopane isomerization ratios.
Diversely sourced degradation products of higher plant lignans were identified in modern and ancient woodrat (Neotoma) middens. The markers indicate extensive chemical modification by intestinal microbial communities of mammals. The observed defunctionalized phenols represent a group of natural products, and their structural elements reveal information about the plant source. The phenols are derived mainly from two precursor types: (1) enterolactone and derivatives from conifer lignans, and (2) 2,3-bis(3'-hydroxybenzyl)butane and related compounds from lignans such as nordihydroguaiaretic acid common in Larrea sp. (e.g. creosote bush).
Controls on oil family distribution in tectonically complicated, nonmarine, petroliferous basins are commonly difficult to isolate because of the varying ages of potential source rocks, the complex assemblage of organic-rich sedimentary facies, and the geographic variability of burial histories. The Turpan-Hami basin of northwestern China is an oil-bearing intermontane basin where stratigraphic, sedimentologic, and geochemical controls are sufficient to address each of these issues independently and to determine how they influence the current distribution and composition of liquid hydrocarbons.Source rock age is one of three major statistically significant discriminators affecting oil family composition. Both Lower/Middle (Lower or Middle) Jurassic and Upper Permian rocks are important source rocks for the basin. A newly developed diterpane biomarker parameter can distinguish Permian rocks and their correlative oils from Jurassic coals and mudrocks and their derivative oils.Source facies is a second key control on petroleum occurrence and character. A variety of biomarker parameters that reflect source rock depositional conditions are indexed to rock samples from interpreted depositional environments. By erecting rock-to-oil correlation models, the biomarker parameters separate oil families into end-member groups: group 1 oils = Lower/Middle Jurassic peatland/swamp facies (high land-plant input, less reducing conditions), group 2 oils = Lower/Middle Jurassic profundal lacustrine facies (high algal input, more reducing conditions), and group 3 oils = Upper Permian lacustrine facies (high algal, stratified, anoxic conditions).Burial history exercises a third major control on petroleum distribution. Source rock maturation modeling can demonstrate that relatively uninterrupted burial in the asymmetrically subsiding northern Turpan-Hami area (Taibei depression) exhausted Upper Permian-sourced rocks by the Late Cretaceous, which led to southward migration of Upper Permiansourced oils (group 3) into Triassic reservoirs of southern and southwestern Turpan-Hami (Tainan and Tokesun depressions). Subsequent to uplift of the central basin thrust that currently partitions Taibei from Tainan, Lower/Middle Jurassicsourced oils were expelled in the Taibei depression by PaleoceneEocene time, which locally charged Jurassic and Cretaceous reservoirs (groups 1 and 2), forming Turpan-Hami's largest oil accumulations in the basin.
Diversely sourced degradation products of higher plant lignans were identifi ed in modern and ancient woodrat (Neotoma) middens. The markers indicate extensive chemical modifi cation by intestinal microbial communities of mammals. The observed defunctionalized phenols represent a group of natural products, and their structural elements reveal information about the plant source. The phenols are derived mainly from two precursor types: (1) enterolactone and derivatives from conifer lignans, and (2) 2,3-bis(3’-hydroxybenzyl)butane and related compounds from lignans such as nordihydroguaiaretic acid common in Larrea sp. (e.g. creosote bush)
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