Diagenesis of Namurian fluvio-deltaic sandstones from the Trumfleet Field, South Yorkshire

Abstract

The Namurian sequence of the Trumfleet Field, South Yorkshire, UK, contains four sandstone units between the Gastrioceras subcrenatum and Reticuloceras gracile marine bands, each deposited in a period between glacio-eustatic marine transgressions. These medium- to coarse-grained sandstone units up to 25 m thick were deposited as multi-channel, upwards-fining distributary channel fills within a predominantly argillaceous deltaic interdistributary and swamp complex. Associated sandstones are crevasse splay and overbank deposits. Four stages of diagenetic alteration can be identified. Early diagenesis (eogenesis) was controlled by the oxidation of organic matter, causing reducing conditions to be established. Pyrite, siderite and calcite are characteristic of this phase. Burial diagenesis (mesogenesis) can be divided into three stages. The first (early burial) was characterized by dolomite and quartz cements, the second (intermediate burial) was controlled by the generation of acidic pore fluids by the thermal maturation of organic matter in adjacent and down-dip shales, and was manifested by dissolution of feldspar and carbonate with the precipitation of kaolinite in secondary pores. The third stage (late burial) was characterized by the precipitation of ankerite, albite and illite cements. Integrating the diagenetic data with the burial history, and modelling the source rock geochemistry of the wells allows the timing of diagenetic events to be predicted within absolute ranges. Early diagenesis took place very near the surface and early burial diagenesis occurred during the Carboniferous, from depths of only a few centimetres below the sediment-water interface to up to 1500 m burial depth at approximately 140 my. Intermediate burial diagenesis took place when the rocks passed through the 80°C isotherm at approximately 140 my. Late burial diagenesis took place during maximum burial prior to Tertiary uplift, when temperatures in excess of 100°C were reached. The present day porosity is secondary, after carbonate and feldspar dissolution. Primary textural factors have little effect on the present reservoir properties, which are controlled by diagenesis.