Impact of oil emplacement on diagenesis in Cretaceous oil sands

Abstract Seventeen thin sections of Cretaceous oil sands from the Neuquen Basin (Argentina), Sergipe-Alagoas Basin (Brazil), Western Canadian Sedimentary Basin (Canada), Junggar Basin (China), Lower Saxony Basin (Germany), Kangerlussuaq Basin (Greenland), Arabian Basin (Kuwait), Chad Basin (Nigeria), Dahomey Basin (Nigeria), Western Moray Firth Basin (UK), Wessex Basin (UK) and Utah (USA) were examined using the scanning electron microscope (SEM) to improve our understanding on how oil emplacement impairs the progress of diagenesis. Our results show that diagenetic processes affecting sandstones prior to oil emplacement include burial/compaction, silica/calcite cementation, calcite replacement of detrital grains/cements as well as the development of silica overgrowth. Most diagenetic processes were inferred to cease upon oil emplacement into the pores of the sandstones, however, diagenetic processes such as the alteration of detrital grains/cements and precipitation of authigenic minerals/metallic compounds were observed to occur after oil emplacement into the pores of the sandstones. Oil was emplaced in some of the studied Cretaceous oil sands at a relatively early stage when the sandstones were not compacted or cemented. Such Cretaceous oil sands were observed to have had anomalously high porosities of above 38% prior to oil emplacement. The only cement observed in these oil sands are the viscous heavy oils (bitumens) associated with them. Upon extraction of these heavy oils, the oil sands collapse into unconsolidated sands. Occurrence of these bitumen supported Cretaceous sands implies availability of migrating oils while some of the Cretaceous sands were depositing in various basins. Oil emplacement occurred in some of the studied Cretaceous oil sands after the sandstones had undergone some diagenetic processes which did not destroy all their pore spaces. Such Cretaceous oil sands were observed to have had moderate to high porosities of 10%–30% prior to oil emplacement, with some of these sandstones showing evidence of silica overgrowth. Emplacement of oil into the pores of such sandstones is believed to have stopped further development of the silica overgrowth that would have led to the total loss of porosity in these Cretaceous reservoir sands. In some of the studied Cretaceous oil sands, oil emplacement occurred when the sands had experienced a long history of diagenetic events leading to almost total loss of porosity. Common diagenetic features observed in such Cretaceous oil sands include sutured quartz grain-grain contacts and quartz overgrowth.