PLIOCENE FLUVIO-LACUSTRINE DEPOSITIONAL SYSTEMS IN THE SOUTH CASPIAN BASIN; RESERVOIR ARCHITECTURE IMPACTS

Nazim Abdullayev¹, Andrew Bowman¹, Neil Piggott², Gregory Riley¹, Matthew Warner^1
1 BP Caspian Sea Ltd, Chertsey Road, Sunbury, TW16 7LN, UK
² BP Russia BU, Chertsey Road, Sunbury, TW16 7LN, UK

A regional depositional model for the Productive Series of the South Caspian Basin (SCB) is proposed. This model is based on integration of regional seismic data (20,000 Kms) with well and outcrop data and provides insights into sediment delivery and reservoir distribution and architecture of the Productive Series in the SCB.

This depositional model can be summarized in five main phases. These phases are interpreted to record differences in the balance between sediment input, water input and tectonic subsidence, and therefore record the long-term trends of these basin scale variables. The shifting balance between these inputs results in differences in reservoir architecture for the various stratigraphic units. Although not discussed here, there are several orders of higher frequency fluctuations in these variables responsible for smaller scale stratigraphic architecture.

Phase 1. Messinian (~5.8 Ma): Base level fall in the Messinian resulted in isolation of the Caspian Sea from the Black Sea and removal of the connection to the global oceans. 

Phase 2. Early Pliocene: The Messinian base level fall led to dramatic incision of the PaleoVolga in the Central Caspian. Integration of drainage systems to this lowered base level resulted in delivery of large volumes of sediments from the Russian Platform and Greater Caucasus to the SCB via the PaleoVolga River. These sediments were deposited in fluvio-lacustrine depositional systems in paleo-structural lows. These strata form the Lower Productive Series (LPS) and include the Kalin, Under-Kirmaky Sandstone (PK), Kirmaky (KS), Above-Kirmaky Sandstone (NKP) and Above Kirmaky Shale (NKG) suites.

Sandstone reservoirs associated with Phase 2 are high quality, but highly variable laterally as a result of pre-existing topography focussing deposition into topographic lows. Onlap of sandstone reservoirs onto paleostructural highs is common. These reservoirs are highly vertically compartmented as a result of numerous lacustrine flooding events. 

Phase 3. Early/Middle Pliocene: The LPS sediments, described above, infilled pre-existing topography creating a low gradient depositional “ramp” over the majority of the SCB. Middle Productive Series (MPS) strata (Pereriv, Balakhanny and Sabunchy Suites) were deposited on this “ramp” and record the high frequency alternation of regionally extensive braided fluvial and shallow lacustrine depositional systems. These high frequency alternations are interpreted to result from climatic cycles that caused dramatic variations in the water and sediment input into the basin. Overall these strata show a dominantly aggradational stacking pattern. The overall aggradational stacking pattern suggests that there is a long-term balance between sediment input, water input (P/E ratio) and subsidence.

Sandstone reservoirs associated with Phase 3 are high quality and very extensive, both laterally and down depositional dip. They are highly vertically compartmented as a result of numerous lacustrine flooding events. These form the predominant offshore reservoirs in Azerbaijan. 

Phase 4. Mid/Late Pliocene: Upper Productive Series (UPS) strata (Surakhany Suite) record a retrogradational stacking pattern and onlap onto the incised PaleoVolga channel. This led to destruction of the depositional “ramp” and creation of a relatively deep water (~100 –200 meters) lake surrounded by lacustrine shelf margins. These patterns are interpreted to result from a reduction of sediment input relative to the water input (P/E ratio). 

Sandstone reservoirs associated with Phase 4 are locally of high quality, but highly variable laterally and show significant down depositional-dip degradation.

Phase 5. Latest Pliocene: Overlying the Productive Series, the Akchagyl Suite, records a major regional transgression and a return to marine conditions in the SCB. The Akchagyl Suite is a regionally extensive condensed section and is downlapped by the Pleistocene shelf margin complexes. These shelf margin complexes are prograding into a deep water lake (~ 1000 meters) that is similar to the present depositional setting of the Caspian Sea.