Fan-wei Meng

Hydrocarbon migration through salt: evidence from Kelasu tectonic zone of Kuqa foreland basin in China

Halite and other evaporates are rarely observed to degrade via brittle faulting because of their ductile properties. Hydrocarbons resulting from fractures and faults are rarely found in salt structures. However, large-scale hydrocarbons resulting from faulting are not well-documented. The Dabei Gasfield, which produces natural gases with few condensate oils, is found under Paleogene evaporates in the Kelasu tectonic zone of the Kuqa foreland basin. Above these evaporates, the Dawanqi Oilfield produces oils with few natural gases in the Kelasu tectonic zone of the Kuqa foreland basin. The biomarker distribution from oils and carbon isotope from natural gases between the Dabei Gasfield below evaporates and the Dawanqi Oilfield above evaporates are very similar, which indicates that they have same hydrocarbon source. The results provide direct evidences of the rare migration of hydrocarbon through salt due to fractures and faults to form oil reservoirs above the salt. This study indicates that hydrocarbons occasionally exhibit widespread migration through salt due to extensive brittle faulting.

Paleogene organic-walled dinoflagellate cysts in the Shulu Sag, Hebei Province, China

Some well-preserved fossil dinoflagellate cysts were found from a lacustrine facies with salt deposition in the Shulu Sag of Hebei Province, China. The diversity and abundance of the assemblage were both low, which indicated a paleoenvironment of brackish water. The main species could be found from the First Member of the Shahejie Formation in Coastal Bohai Region of China, which suggested a geological age of Oligocene. Fossil dinoflagellates showed significance in non-marine facies for recovery of paleoenvironment and regionally stratigraphic correlation. Brackish dinoflagellates are constructive to the formation of salt rocks, and dinoflagellate cysts in salt lakes are easy to be preserved as fossils.

Clay minerals from rock salt of Salt Range Formation (Late Neoproterozoic–Early Cambrian, Pakistan)

The clay minerals of Late Neoproterozoic–Early Cambrian rock salt of Salt Range Formation of Pakistan have been studied by means of X-ray diffraction, scanning electron microscopy and dispersive X-ray spectrometry, complex thermal and chemical analyses. The clay minerals association of pelitic fraction of water-insoluble residue of these deposits consists of corrensite, chlorite and illite with the admixture of unordered mixed-layered chlorite–corrensite and chlorite–smectite; in some samples, the admixture of smectite occurs. The expandable layers in corrensite are determined as smectite. In studied samples the chlorite, corrensite and mixed-layered species are presented by trioctahedral Mg-rich type and illite is dioctahedral and enriched by Fe; this association of clay minerals is typical for evaporite deposits. Transformation of clay minerals proceeded under the impact of several factors of different direction and intensity. In evaporite basin, the elevated salinity of brines reinforces the processes of clay minerals structure ordering causing disappearance of mixed-layered minerals and thus decreasing the number of clay mineral species; in the brines originated from SO4-rich seawater, the clay mineral associations are richer comparing to Ca-rich brines. Local factors—volcanic ash input, elevated content of organic matter slow down the transformation processes thus also increasing the number of clay mineral species. We explain the unexpectedly rich clay mineral association (as for the halite stage of evaporation) in studied rocks by the strong effect of local factors.

The Yudomski event and subsequent decline: new evidence from δ 34 S data of lower and middle Cambrian evaporites in the Tarim Basin, western China

Marine evaporitic sulfates (gypsum and anhydrite) can record ancient seawater sulfur isotopic data; however, these records are scarce and widely dispersed owing to both restricted environments in which they form and their propensity to be eroded. The late Neoproterozoic–early Cambrian transition was a pivotal timeframe in Earth’s history, witnessing the early evolution of animal life and major environmental changes. Seawater chemistry changed abruptly during this interval, including significant changes to the sulfur cycle as evidenced by unusually high sulfur isotopic values. This positive sulfur excursion, termed the Yudomski Event, has been reported previously from early and middle Cambrian units in Siberia, Iran, Australia, and India. In this study, we provide the first report of the Yudomski Event in early and middle Cambrian evaporites from the Tarim Basin, northwestern China. Our data support this event having been of global significance. We additionally report early and middle Ordovician sulfur data from China, which constrain the decline of the Yudomski Event to the late Cambrian.

Stromatolites in Middle Ordovician carbonate–evaporite sequences and their carbon and sulfur isotopes stratigraphy, Ordos Basin, northwestern China

At the present day, stromatolites are very rare and limited to high-salinity settings. However, abundant and variable stromatolite fossils occur in thick evaporite sequences, the Middle Ordovician Majiagou Formation of the Ordos Basin, northwestern China. The setting and fossil assemblage imply that the high salinity prevented metazoan grazing and allowed stromatolites to flourish. The carbon isotope curve based on new data (average data range from −4.52 to +0.56‰) from drilling cores in Majiagou Formation is in good agreement with Middle Ordovician (Darriwilian), and supports the Middle Ordovician age inferred from biostratigraphy. Sulfur isotopes values (range from +27.1 to +28.0‰) are higher than those of the Upper Ordovician but much lower than those of the Lower Middle Cambrian; thus, the sulfur values imply that the Majiagou Formation is of Middle Ordovician age. New carbon and sulfur isotope data imply that these strata are of Middle Ordovician age, in good agreement with previous paleontological dating.