No abstract

This study focuses on low resistivity thick layer sandstone in the X∼XII groups of the third member of Qingshankou Formation at Daqingzijing oilfield, along with comprehensive data of logging, core, oil test, and production test. Based on the current data, we characterized the logs of low resistivity thick-layer sandstone, quantitatively identified calcareous sandstone and low resistivity reservoir, predicted the reservoir thickness, and further explored the causes of low resistivity reservoir of the region. The resistivity of thick layer sandstone in the X∼XII groups of Qingshankou Formation can be classified into low amplitude logfacies, middle amplitude logfacies, and sharp high amplitude logfacies. Sharp high amplitude logfacies sandstone is the tight sandstone of the calcareous cementation. Low amplitude logfacies sandstone is water layer. For the middle amplitude logfacies sandstone, water layer or oil-water layer can be identified with the identification standard. Low amplitude structure, high clay content, high irreducible water saturation, and high formation water salinity are attributed to the origin of low resistivity oil layer.

Deepwater oil and gas exploration has become a global hotspot in recent years and the study of the deep waters of marginal seas is an important frontier research area. The South China Sea (SCS) is a typical marginal sea that includes Paleo SCS and New SCS tectonic cycles. The latter includes continental marginal rifting, intercontinental oceanic expansion and oceanic shrinking, which controlled the evolution of basins, and the generation, migration and accumulation of hydrocarbons in the deepwater basins on the continental margin of the northern SCS. In the Paleogene, the basins rifted along the margin of the continent and were filled mainly with sediments in marine-continental transitional environments. In the Neogene-Quaternary, due to thermal subsidence, neritic-abyssal facies sediments from the passive continental margin of the SCS mainly filled the basins. The source rocks include mainly Oligocene coal-bearing deltaic and marine mudstones, which were heated by multiple events with high geothermal temperature and terrestrial heat flow, resulting in the generation of gas and oil. The faults, diapirs and sandstones controlled the migration of hydrocarbons that accumulated principally in a large canyon channel, a continental deepwater fan, and a shelf-margin delta.

In the Carboniferous–Permian period, several organic-rich black shales were deposited in a marine–continental transitional environment in the Linfen area on the eastern margin of the Ordos Basin. Integrated sedimentological and organic geochemical analyses are performed on an outcrop in order to clarify the relationship between paleoenvironment and organic matter accumulation. The results of this study show that the marine–continental transitional strata of the Upper Carboniferous Benxi Formation to Lower Permian Taiyuan and Shanxi Formation exposed in the Linfen area are composed of sandstone, shale, coal, and limestone. Total organic carbon (TOC) contents of the studied samples were mainly distributed in the range of 0.59%–35.4%, with an average of 7.32%. From Benxi Formation to Shanxi formation, the humidity gradually increased, and the climate gradually changed from hot and humid to warm and humid during Carboniferous to Permian. The deposition of the Shanxi Formation ended with the climate returning to hot and humid, having an oxic-suboxic conditions and a high paleoproductivity. Paleoredox conditions and paleoproductivity are the two vital factors controlling the formation of organic matter in black shales. The transitional environment characterized by oxic-suboxic, relatively high deposition rate, and various source of organic matter, although different from the marine environment, provides a good material basis for the deposition of organic-rich shales.