Lower Paleozoic Marine Research Articles

Geochemical Characteristics and Origin of Natural Gas in Tarim Basin, China

The Tarim basin, located in northwest China, is one of the largest basins in the world that has not been well explored. In recent years the large scale of petroleum exploration in the basin has led to the discovery of a series of oil and gas fields with an approximate 1:1 ratio of oil to gas resource. This means that the natural gas resource is very important for the hydrocarbon resource in the Tarim basin. In this basin most of the discovered gases are condensate-associated gas and oil-associated gas. Research on chemical components and isotope composition of carbon and hydrogen of natural gases from the Tarim basin shows that all discovered gases in this basin are thermogenic. With respect to source organic matter, there are two types of gases: (1) oil-type gas generated from Sinian to lower Paleozoic marine source rocks with the sapropelic organic matter, with -44.5 to -33.8o/oo of d13C1, -42.0 to -28.1o/oo of d13C2, -35.4 to -28.4o/oo of d13C3, and heavier than -200o/oo of dD1 and (2) coal-type gas generated from Mesozoic terrestrial source rocks with the humic organic matter, with -40.5 to -33.1o/oo of d13C1, -29.7 to -21.3o/oo of d13C2, -26.3 to -20.3o/oo of d13C3, and lighter than -200o/oo of dD1.

Genetic type and maturity of Lower Paleozoic marine hydrocarbon gases in the eastern Tarim Basin

Hydrocarbon compositions and their carbon isotopic data for 94 natural gas samples were combined with the results of thermal simulation experiments to complete a comprehensive study of maturity, genetic types and sources of Lower Paleozoic marine hydrocarbon gases from the eastern Tarim Basin. All of the Lower Paleozoic gases are relatively dry (dryness coefficient >0.9) and belong to the marine sapropelic type of cracked gases. These gases are characterized by low δ 13 C 1 and low Δ(δ 13 C 2−δ 13 C 1) values due to a lack of terrestrial source input. Thermal simulation experiments on Upper Proterozoic to Lower Paleozoic source rocks resulted in a δ 13 C 1–R o regression equation that was used to discriminate the maturities and sources of the natural gases. These natural gases are divided into four genetic types: (1) humic, coal-derived pyrolytic gas; (2) lacustrine sapropelic oil-cracking gas; (3) the marine sapropelic, highly mature oil-cracking gas; and (4) overmature oil-cracking gas. Types (1) and (2) were mainly generated from the Upper Triassic-Middle and Lower Jurassic; types (3) and (4) are mainly generated from the Lower Paleozoic. The data can be used to describe multiple sources and multiple generating stages of natural gas.

The discovery of retene in Precambrian and Lower Paleozoic marine formations

Microscopic and SEM observations of thin sections and kerogens and GC-MS analyses of aromatic fractions in sediment extracts and pyrolysis products of kerogens provide strong evidence suggesting that in the Lower Yangtze Valley area the Sinian and Lower Paleozoic carbonate formations of marine facies received no higher plant input, but there was produced abundant retene during diagenesis and abietic acid substances are still its procursors.

Structural Style and Petroleum Prospects of the Kuqa Depression, Northern Tarim Basin, Northwest China: ABSTRACT

The Kuqa depression is a 400-km {times} 100-km, east-west-trending foredeep basin located on the northern margin of the Tarim craton in northwestern China. A 10-km-thick nonmarine Mesozoic and Cenozoic sedimentary section has accumulated in the basin as the Tian Shan range to the north and has been episodically uplifted and thrust southward, tectonically loading the craton margin and providing source areas for clastic sediments. Regional mapping from Landsat images and field reconnaissance reveal important details of the structural style of the Kuqa depression. The Kuqa depression is characterized by thin-skinned deformation resulting in a series of steep, faulted, elongate folds. Thick, mobile Tertiary shale sequences and an Upper Cretaceous-lower Tertiary evaporite section in the west form detachment horizons separating disharmonically deformed structural packages. At the southern margin of the depression, the Mesozoic-Cenozoic section overrides faulted Paleozoic strata of the North Tarim uplift. Here, the Neogene Qiulitage anticline extends 250 km along the basin margin. Beginning in 1984, several important oil discoveries have been made in the Paleozoic section on the North Tarim uplift. Analysis of an oil sample indicates its compatibility with a lower Paleozoic marine source. Other potential source rocks in the Kuqa depression include Jurassic oil shales andmore » coals. Sandstone reservoir strata and structural trapping geometries are numerous, but exploration risks are complicated by the presence of the ductile evaporite and shale seals. If nonmarine source rocks are also present in the thick Cenozoic section, the Kuqa depression may be an attractive exploration target.« less

Possible effect of readily available iron in volcanic ash on the carbon to sulfur ratio in lower Paleozoic normal marine sediments and implications for atmospheric oxygen

Near-surface rates of microbiological sulfate reduction in most sediments often exceed rates of iron release, which consequently limit the precipitation of early diagenetic iron sulfides. Excess hydrogen sulfide produced by sulfate reduction generally escapes from the sediment before it can be fixed as iron sulfide; however, the efficiency of hydrogen sulfide fixation may be significantly increased by the presence of metastable, easily dissolved iron-bearing phases, such as volcanic glass. Sediments that efficiently trap hydrogen sulfide may have lower ratios of carbon to sulfur than those typically found in normal marine environments. The abundance of volcanic ash in sediments related to the closing of the proto-Atlantic may account for the low ratios of carbon to pyritic sulfur in lower Paleozoic sediments from Europe and the United States compared to equivalent sediments from Australia. The increase in the efficiency of trapping of hydrogen sulfide does not affect the amount of organic matter oxidized or sulfate reduced. It does, however, decrease the amount of sulfide that escapes the sediment and is reoxidized to sulfate. The net effect is a decrease in oceanic sulfate and an increase in atmospheric oxygen.