Dating diagenesis in a petroleum basin, a new fluid inclusion method

Abstract

The final porosity and permeability of sandstone petroleum reservoirs is greatly affected by the diagenetic growth of minerals after deposition. For example a sand may be deposited with a porosity of 25% and a permeability of 5,000 mdarcy (mD)¹; diagenetic growth of quartz around detrital sand grains may leave a rock with only 10% porosity, and later growth of clays may partly fill these remaining pores and block inter-pore connections, reducing permeability to 100 mD (ref. 2). If the depth and timing of such diagenetic alteration can be measured and the extent of diagenesis estimated, then prediction of the diagenetic state of undrilled sandstones may become possible and diagenesis related more closely to the timing of hydrocarbon migration and the formation of hydrocarbon traps. We present an example of a new method for estimating the date of quartz diagenesis using a combination of techniques from thin section petrography, fluid inclusion thermometry, organic geochemical thermometry and sedimentary basin stratigraphic analysis. These results suggest that quartz in the Beatrice oilfield was precipitated from moving and cooling pore fluids, at a temperature between 68 °C and 94 °C in the late Jurassic.