A Lacustrine Petroleum System: The Pliocene South Caspian Basin, Azerbaijan

Gregory W. Riley, Nazim Abdullayev, Tony Reynolds, Toby Harrold, Stephan Duppenbecker, and Andrew Bowman
BP, Sunbury-on-Thames, United Kingdom

Exploration and production data from the South Caspian Basin, Azerbaijan highlight the importance of regional-scale facies (permeability) architecture as a primary control on the petroleum system. The Pliocene of the South Caspian Basin was deposited on a low gradient ramp that fed into a large, shallow-water lacustrine basin. Fluctuations in lake level and sediment supply resulted in a strongly layered sedimentary sequence at a scale of meters to tens of meters, with extensive fluvio-deltaic sandstone “sheets” interbedded with extensive lacustrine shales. Gradual basinward thickening is accompanied by an increase in the proportion of lacustrine shales, a decrease in sandstones and compaction disequilibrium induced overpressure. Late Pliocene uplift and folding resulted in creation of large (> 2km relief) anticlines which result in local surface exposure of reservoir intervals.

The above geologic setting strongly controls the critical risk factors of the petroleum system.

1. Extensive sandstone “sheets” allow for significant lateral migration of hydrocarbons towards the basin margin. 2. Regionally extensive lacustrine shales form excellent seals, capable of holding hydrocarbon columns in excess of one kilometre. 3. Regionally extensive sandstone “sheets” allow for regional transfer of pressures from the overpressured basin to basin margins, resulting in decreased basinal reservoir pressures and enhanced seal capacity. 4. Subregionally extensive sandstone “sheets” allow for subregional, syncline to crest, transfer of overpressures and therefore elevated crestal sandstone pressures, reduced seal capacity for petroleum accumulations and stacked pays. 5. The stongly layered reservoir and seal sequences result in vertically-segregated and laterally-connected reservoir systems.