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Top Down Petroleum System Analysis: Exploiting Geospatial Patterns in the Properties of Hydrocarbon Fluids

Abstract

The fluid-phase and bulk properties of petroleum fluids are controlled by the source rock organofacies, maturation, expulsion and pressure and temperature along migration pathway and in the traps. Together with the basin geometry and framework, these factors dictate the spatial distribution of oil and gas. “Top-down” petroleum systems analysis is the systematic interpretation of the distribution and properties of fluids, along with shows, seeps, dry holes, and any other relevant well data in the geological context. The aim is to discern patterns and place them in a petroleum system framework, thereby improving the quality of pre-drill prediction.The availability of “big-data”, especially the copious production data from unconventionals, and data analytics tools have enabled recognition of spatial patterns in fluid phase and properties: API gravity, GOR and the interpreted maturity of oils tend to be lower near the basin margins, while gas-condensates are most often found near the basin center, partly due to maturity variation but also to “migration lag” effects. In vertically drained systems, such as deltas and rift basins, lower maturity fluids are found in shallower/younger stratigraphic units. GOR and API gravity both increase with depth but can reverse locally in a leak through system.Phase separation also exerts a significant control on fluid phase and properties, especially in a mixed oil and gas petroleum system typical of deltaic settings. In many cases, GOR and CGR are controlled simply by reservoir pressure as the saturation pressure has already been reached along the migration pathway. At the same time, fluid phase found in the trap depends on whether the trap leaks or spills. High GOR (volatile) oils can only exist as a single phase in deep reservoirs due to their high saturation pressure. In unconventional settings, migration and/or pressure reduction may cause a moderate GOR oil to reach bubble point and then produce anomalously high GOR from a reservoir where the rocks have only low local thermal maturity.In this paper we show several examples of top-down petroleum systems analysis from around the world. As we often find fluids before we drill the actual source rock, this methodology can help constrain the petroleum system at an early stage and provide a reality check for basin models.