Lacustrine basin hydrocarbon exploration - current thoughts

Although much of the world's petroleum resource-base is associated with mar ine systems, regionally lacustrine petroleum systems are important. Individ ual accumulations may exceed several billion barrels. In each of these case s the oil is derived from a lacustrine source rock and may be produced from either nonmarine or marine reservoir rocks. The purpose of this paper is t o describe the factors that control lacustrine source rock development and the nature of lacustrine reservoirs. Lacustrine oils display different phys ical and chemical characteristics than their marine counterparts. These dif ferences can be related to the nature of their precursor material. Although the nature of the products are different, the geochemical threshold criter ia for defining source rocks in both settings are the same because of commo n expulsion requirements. Commercially significant lacustrine systems requi re the presence of large, long-lived lakes. Such lake settings are tectonic in origin and restricted to climatic settings where precipitation exceeds evaporation. Within these large lake systems three primary factors determin e source rock potential and quality. These factors are primary productivity level, organic preservation potential, and matrix sedimentation rate, whic h controls the dilution of preserved organic matter. Source rock potential is maximized where both productivity and preservation potential are maximiz ed and sedimentation rate is minimized. To some degree these factors can co mpensate for each other. Hydrocarbon reservoir potential within lacustrine basins is partially impacted by overall tectonic setting. Within extensiona l settings, transport distances tend to be limited, with much of the sedime nt being transported away from the basin. The sediments delivered to the la ke are poorly sorted and sedimentologically immature, commonly resulting in poor reservoirs due to both primary properties and their susceptibility to diagenesis. Within rifts better reservoirs tend to develop along platform or flexural margins. Stacking of reservoirs is important in lacustrine syst ems but baffles and barriers are often present between individual sand unit s. These barriers form as a result of lake level fluctuations. In compressi onal settings transport distances tend to be longer, resulting in more matu re, better sorted sediments leading to higher quality reservoirs. These res ervoirs typically develop in fluvial-deltaic and wave-dominated shoreline s ettings. Lacustrine carbonate reservoirs are locally important. These carbo nates tend to develop during lake level lowstands and are dependent on diag enesis (dissolution and karstification) for porosity and permeability devel opment. Lacustrine reservoirs are often stratigraphically and areally limit ed and display low individual well production rates. Within 'pure' lacustri ne systems exploration opportunities appear to be often restricted by eithe r reservoir presence or quality (i.e., production rates). The best explorat ion opportunities in lacustrine basins appear to be associated with hybrid systems where a lacustrine source and marine reservoir exist.