David Zinniker

Upper Oligocene lacustrine source rocks and petroleum systems of the northern Qaidam basin, northwest China

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.

Lower-Middle Jurassic Nonmarine Source Rocks and Petroleum Systems of the Northern Qaidam Basin, Northwest China

The Qaidam basin is a nonmarine, petroliferous basin on the northeastern margin of the Tibet Plateau. Potential source rocks are reported from Tertiary saline lacustrine deposits and Jurassic freshwater lacustrine or terrestrial strata. The Jurassic source rock is similar, in terms of depositional system, lithology, age, and organic geochemistry, to nonmarine Jurassic rocks that are thought to be potentially significant hydrocarbon sources in many basins of central Asia, including the Tarim, Junggar, and other important petroleum-producing basins. These Jurassic source rocks were deposited in freshwater lacustrine systems in intracontinental foreland-style basins during the Early and Middle Jurassic. These lacustrine strata are associated with coals and coaly mudstones with possible secondary importance as hydrocarbon source rocks. Oils from the northern Qaidam basin can be divided into two groups based on molecular indicators of depositional environment and age-diagnostic biomarkers: those derived from this Jurassic freshwater lacustrine source rock and those derived from a Tertiary hypersaline lacustrine source rock. The correlation of some Qaidam oils to a Jurassic source establishes a petroleum system involving these nonmarine Jurassic source rocks. Oils of this petroleum system have been produced from Tertiary and Mesozoic reservoirs in anticlinal and thrust-related traps in northeastern Qaidam. Maturation modeling of the Jurassic source rock indicates that these oils were generated and expelled during the Miocene-Pliocene in much of northern Qaidam. The geological context of the basin suggests that drainage within the basin primarily was vertical, probably mainly along faults, until evaporite or overpressured shale seals were encountered in the overlying Cenozoic section. Documentation of this petroleum system suggests other exploration targets remain underevaluated in northern and southwestern Qaidam; furthermore, the northern Qaidam petroleum system is a useful analog to interpret the role of similar Jurassic source rocks in other, sparsely documented, basins of central Asia.

Application of new diterpane biomarkers to source, biodegradation and mixing effects on Central Llanos Basin oils, Colombia

Llanos Basin oils have been commonly attributed to the prolific Upper Cretaceous age source formations outcropping in the Eastern Cordillera. Recently, however, variable oleanane indices have been reported for Llanos oils, suggesting a contribution from Tertiary source sequences largely on the basis of the presence of high relative abundance of oleanane, a Tertiary age-diagnostic biomarker. A homologous series of 25-norhopanes in many central Llanos Basin oils indicates that heavy biodegradation is particularly common to the initial oil charging reservoirs from Upper Cretaceous marine sources. One homologue, 20S-25-norhomohopane [C30 25-norhopane], co-elutes with oleanane and may thus contribute to the peak attributed to oleanane. Also, confusing the source identification is the possibility that some Cretaceous facies also contain oleanane. Three plant diterpanes, which may be isomers of fichtelite and are common to the Central Llanos oils, have been observed in an Eocene source rock sample, but not in Cretaceous rocks or oils, providing additional strong evidence for a Tertiary contribution to some Llanos Basin oils. In mixed oils with low to moderate oleanane contents and with a strong, co-eluting C30 25-norhopane, the fichtelite isomers are the more reliable indicators of Tertiary source input.

Biogeochemical evidence for the presence of the angiosperm molecular fossil oleanane in Paleozoic and Mesozoic non-angiospermous fossils

Abstract Recent molecular phylogenetic and molecular clock data both suggest a pre-Mesozoic age for the divergence of the angiosperm lineage from other seed plants, greatly predating the confirmed fossil record of the angiosperm crown group. In addition, molecular phylogenetic studies have not supported the morphologically based conclusion that gnetophytes are the extant sister group to angiosperms. We examine these relationships and divergence ages by using a novel approach of examining the presence of oleanane. This includes the development of methods using zeolites to preferentially reduce hopanes that can co-elute with oleanane. The presence of this molecular fossil strongly correlates with angiosperm diversification; in its functionalized form, along with its triterpenoid precursors, it is found in many living angiosperms. Our data show that among non-angiosperm seed plants examined thus far, oleanane is found only in fossil Cretaceous Bennettitales and Permian Gigantopteridales, both of which share characteristics with angiosperms. Previous morphological phylogenetic results indicate Bennettitales could be a sister group to or member of the angiosperm stem lineage, and results of our preliminary phylogenetic analysis including the Gigantopteridales suggests the same. Our data, based on a new pyrolysis method to treat living species, support previous research indicating that oleanane and its precursors are absent in living gnetophytes. If oleanane originated once in seed plants then the angiosperm stem lineage would have diverged from other seed plant lineages by the late Paleozoic.

Geochemical characteristics and correlation of oil and nonmarine source rocks from Mongolia

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.

Controls of oil family distribution and composition in nonmarine petroleum systems: A case study from the Turpan-Hami basin, northwestern China

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-Hamis largest oil accumulations in the basin.