Meteoric freshwater leaching and its significance to reservoir quality in a buried hill of lower-middle Jurassic fluvial sandstones: A case study from the Huanghua Depression, Bohai Bay Basin, China

Abstract

Within polycyclic sedimentary basins, meteoric freshwater leaching plays an important role in the formation of secondary porosity. Whereas, when superimposed by other late diagenesis processes, it is often difficult to identify its diagenetic indicators. The Jurassic clastic reservoirs in the “Wmy buried hill” of the Huanghua Depression testify from such meteoric freshwater diagenesis. The “buried hill” is named after a village called “Wmy” and nowadays buried deeply into a Paleogene lacustrine sag, which displays like a highland or hill. These reservoir sandstones were studied based on a variety of approaches including 3-D seismic interpretation, core and thin section observations, X-ray fluorescence analysis, Electron Probe Microanalysis and stable isotope analysis, combined with reconstruction of the burial and thermal histories. At last, the evolutionary model of reservoir quality was addressed. Difference of sedimentary fabrics in sand bodies leads to the heterogeneity of pores distribution, making the permeability in coarse-grained and fine-grained sandstones largely different. The meteoric water leaching caused the loss of Fe and Ti, leading to a zonation of core colors with white and red zones. In addition, the dissolution by meteoric water of biotite and volcanic fragments as well as feldspar grains produced large amounts of secondary porosity without formation of diagenetic by-products, which thus points to an open diagenetic system. The calcite cement of meteoric freshwater origin displays relatively low Fe and high Mn content exhibiting a low Fe/Mn ratio. The origin of Mn is probably dissolution of volcanic fragments. The evolutionary process of reservoir quality can be divided into four stages. The second stage is the crucial period for formation of secondary porosity whilst faults derived from the compressional stress during tectonic uplif associate coal-bearing source rocks with Jurassic reservoirs. Moreover, the last stage is the critical period for petroleum and/or gas migration and accumulation because the underlying coal-measures entered the hydrocarbon generation window and overlying Paleogene sealing rocks were intensely compacted. Scale of potential reservoirs in the whole well profile are evaluated by statistical methods, the total thickness of which can reach about 25 m. Reservoirs close to the unconformity surface between the Mesozoic and Cenozoic display more effective porosity than the deeper ones, which should be the target for the subsequent hydrocarbon exploration.