RESULTS OF PALEO VOLGA DELTAIC SEDIMENTS STUDY WITHIN GUNASHLY FIELD

E. Aleskerov¹, A. Narimanov¹, B. Husseynov², S. Jafarova¹, O. Akhmedov²¹ State Oil Company of Azerbaijan Republic; 73, Neftchilar Ave, Az 1004, Baku, Azerbaijan
² BP Azerbaijan, Villa Petrolea, 2 Neftchilar Ave, Az 1003, Baku, Azerbaijan

Subsidence of the South-Caspian oceanic bed in the late Miocene and earlier Pliocene resulted in sharp sea bed fall and deposition in the basin of rather thick deltaic and lacustrine sediments that include several high porosity sand horizons of Middle Pliocene.

Oil and gas generation in the region, most probably, is connected with Maikopian shales of Oligocene-lower-Miocene period that were petroleum source rocks and caused oil generation during the last 3 million years and gas generation during the last 0,5 million of years.

After migration oil was accumulated in the structural traps, that are mainly large anticlines, formed in the vertical as well as in the lateral directions along the uplift strikes of the initial simple faults.

Predominant source in sand deposition formation of the productive pay sections of Absheron threshold was N and NW paleoflow sediments from Paleo-Volga during the Pliocene.

Main sand reservoirs of Guneshly field were deposited in the fluviodeltaic and shallow marine environments. Intervals of sand depositions are regionally separated by correlated shale boundaries that reflect the transgression surfaces. The presence of authigenic glauconite, carbonates and pyrite in core samples is consistent with depositional conditions of shallow marine setting. Sedimentation process was highly dynamic with Caspian water level changes.

Sand layers of Balakhany and Pereriv horizons were deposited from the same sedimentation source - Paleo Volga flow during the transgression caused by the relative highstand.

Sand bodies in both horizons that have grains fining upwards and sharp bases can represent delta channels deposited in shallow marine settings. However, waves, tides and climatic as well as other factors could not but affect the sedimentation environments. This leads to the change of sedimentation environments even in small areas between single wells. Delta deposition setting becomes non-characteristic for delta. Distribution channels that hit the deposited earlier delta environments leave fluvial indications in depositional structure, distribution of grain sizes, what is reflected ultimately in log curves. A progressive increase in marine sedimentation in these horizons with sediments being actively delivered into the basin where separate distribution channels flowed into the Caspian Sea may have occurred as shallow water mouth bars and shoals.

Insignificant changes of sea levels due to the subsidence, supply of inflow or eustasy control flooded existing depositions and originated estuaries environments of which are associated with formation of transgressive shorelines and depositions of shallow marine bars.

Correlation work and layering of the both Balakhany and Pereriv horizons were based mainly on well log data analysis and results of their interpretation. To work out the scheme of horizon layering a cyclo-stratigraphic approach used in sequence stratigraphy was taken.

Cyclo-stratigraphic analysis of the field log data was based on the identification of regional unconformities and regional transgression surfaces that can be correlated between wells rather accurately. Unconformities were identified by significant breaks in log profiles and mark the bases of large-scale basin deepening. Each cycle is interpreted to represent a sequence, and unconformities are interpreted as sequence boundaries.

Transgression surfaces and correlated boundaries of maximum flooding surfaces are clearly identifies in log curves. In Gunashly field within Balakhany and Pereriv horizons sedimentation cycles are bounded by the maximum flooding surfaces.

Within these cycles sand bodies can be interpreted according to the depositional settings as sands of the delta channels, channel sandstones, shallow water mouth bars and shoals. Maximum flooding surfaces are regionally represented by the correlated shale intervals.

Sand bodies have a good lateral connection and their reservoir properties have different range. There are deterioration tendencies of reservoir properties of the horizons upwards the section on the account of total pattern of basin deepening and processes connected with it.

Figure. Gunashly field channel sand distribution.