The age and dolomitization processes in the Paleo-oil reservoir zone, which is composed of massive dolostones found in the Qiangtang Basin (SW China), are still debated. In this research, the Long’eni-Geluguanna Area was selected. Macroscopic information, thin sections, and geochemical methods were used to investigate the dolomitization characteristics and the processes that controlled dolomitization. Five types of replacive dolomites and two types of dolomite cement were observed. Some of the dolomites displayed ghosts of primary sedimentary structures. Saddle dolomites were prevalent, occurring in the interparticle and moldic pores of the limestone which should have been filled at an early diagenetic stage. Ten microfacies types were identified. The foraminifera assemblage provides evidence that the studied interval is of Early Jurassic age. The δ¹³C values are similar to the contemporaneous seawater signature. The REE+Y patterns of limestones and dolostones exhibit similarities to that of seawater. The mean Na and Sr values are comparable to those of other near-normal seawater dolomites. The δ¹⁸O values of all lithologies are markedly depleted. The dolomitization started penecontemporaneously, with deposition. A general sand shoal setting with patch reefs developed. The dolomitizing fluids, near-normal seawater, was probably formed by slight evaporation on top of the shoal. Saddle dolomites in the interparticle and moldic pores might indicate hydrothermal activity, which also caused the recrystallization of some pre-existing dolomites. The recrystallization might have slightly increased the crystal size, demolished the ghost structures, formed saddle dolomites, and altered the REE+Y patterns. The recrystallization extent diminished with increasing distance from the fluids-providing fracture. Furthermore, the existence of protected areas within the sand shoal settings could enhance the vertical and horizontal heterogeneity of dolostone reservoirs.

Dolomite is widespread, but its formation remains debated, including the origin of the Upper Jurassic dolostones in the Franconian Alb, southern Germany. This study investigates a Kimmeridgian dolostone-bearing succession using field observations, microscopy, X-ray diffraction, stable carbon and oxygen isotopes, and trace element data. Dolomite abundance generally increases upward, from partially dolomitized basinal carbonates and marls to pervasively dolomitized reefal, massive facies. In the partially dolomitized interval, dolomite occurs preferentially in marls, matrix, and open or sediment-filled voids. Near and within the lower part of the massive facies, clay-rich dolostone type 1 (DS1) contains finely crystalline dolomite (FD) and medium crystalline dolomite type 1 (MD1), both characterized by lower stoichiometry and crystallinity, whereas clay-poor dolostone type 2 (DS2) in the upper part contains medium crystalline dolomite type 2 (MD2), characterized by higher stoichiometry and crystallinity. Petrographic relationships indicate that dolomitization began during very shallow burial, before marls and, locally, limestones were lithified. REE-Y patterns, together with Na and Sr concentrations, indicate that the dolomitizing fluids were near-normal seawater, consistent with published fluid inclusion salinities from adjacent areas. The fluids are interpreted to have migrated downward and basinward from shallower-water areas of the carbonate system. This hypothesis accounts for the observed facies-dolomitization arrangement in a depositional framework characterized by a general shallowing-upward trend and massive facies expansion. These dolostones are therefore comparable to other Upper Jurassic dolostones commonly associated with evaporation.