Mass transfer between sandstones and interbedded mudstones: Impact on petroleum charge, Bohai Bay Basin, China

Abstract

The Permo-Carboniferous sandstones in the Gubei area, Bohai Bay Basin, are reservoirs for large accumulations of natural gas. The natural gas accumulations only occur in the sandstone beds thicker than 1.9m. Moreover, the maximum porosity of every bed correlates positively with bed thickness up to 2.0m and for thicker beds porosity is uniform. Porosity transitional zones developed at the top and bottom of each bed, with mean thicknesses of 1.25m and 0.75 m, respectively. Porosity shows a positive correlation with the distance to the sandstone/mudstone contact in the zones. Interpretations based upon an extensive petrographic and geochemical database indicate that the sandstones experienced a mass fluid input from the adjacent mudstones during the early diagenesis. The infiltration resulted in extensive clay coats and pore-filling cements in sandstones, which were effective for inhibiting dissolution of grains during subsequent diagenesis process. The mass introduction only affected the marginal parts of the sandstones within 1.25 m of the top and 0.75m of the bottom of every bed, causing thin sandstone beds with thickness of approximately 2m to be tightly cemented totally. Thus the central parts of each (thicker) beds became preferential sites for the natural gas accumulation. The differential levels of cementation in thin and thick sandstone beds observed here has

Coupling between tectonic activity and diagenetic evolution of a clastic buried hill—a case study from Gubei low buried hill in Jiyang Depression of Bohai Bay Basin

“…Previous studies have shown that the structural evolution of Gubei low buried hill can be divided into three stages: compressive reverse fault in the Indosinian period, extensional fault depression in the Yanshanian period, and the transition from compressive to extensional structure in the Yanshanianearly Himalayan period (Hu et al 2019). During the Indosinian period, the Yangtze plate subducted northward and collided with the North China plate.…”

“…In Gubei low buried hill, there is a set of meandering river sedimentary system in the Upper Shihezi Formation (Hu et al 2019). A huge amount of reservoir sand bodies has developed on the microfacies of meandering banks and the remaining sedimentary microfacies on the bottom of rivers in the Kuishan section, the middle of the Upper Shihezi Formation.…”