D. Tambrea, A. Raileanu, V. Borosi, and V.Sindilar

Romanian Oil Corporation Petrom S.A., Geological Exploration Research and Design Center, Bucharest, Romania

ABSTRACT: Seismic Facies and Depositional Framework of Eocene Deposits, in Central Romanian Black Sea Offshore. Implications for Hydrocarbon Exploration

The study area comprises the Eocene Istria Depression bounded by Peceneaga - Camena and Heracleea faults, in front of North Dobrogea Orogene and Central Dobrogea, in the Romanian Black Sea offshore. Location, size, facies composition of the Eocene deposits are primarily determined by the tectonic setting. Part of the Cimmerian extensional faults were inverted due to the regional polyphasic Alpide compression. Beginning with the Paleocene, a discrete phase of strikeslip deformation component occurred induced by Peceneaga - Camena master fault rejuvenation.

There are four main local Eocene depocenters/depressions: Iris - Venus, West Lebada, East Lebada - Minerva and Histria, that younged and became more active from southeast to northwest, developing in an en echelon manner. The Eocene deposits were partly removed during the Oligocene northwest uplift and southeast submarine erosion. The Eocene sedimentation was also influenced by eustatic changes and sediment supply generating the following successions: a proximally uplifted depositional sequence type 1 (LSW, HST) with oblique - sigmoidal offlap geometry during the Middle Eocene and two consecutive depositional sequences type 1 (LSW, TST, HST) with sigmoidal offlap architecture.

The analysis of 100 two - dimensional (2D) seismic lines and two - three dimensional (3D) seismic surveys and well data (cores, cuttings, logs, VSP) has permitted to outline that the geodynamic evolution of Eocene basins has led to the deposition of carbonate - siliciclastic turbidite, porous carbonate shelf margin reservoir deposits, and muddy carbonate turbiditic source rocks that present new Eocene petroleum targets.

Eocene deposits are firstly represented by coarse grained carbonate - siliciclastic facies with Cibicides westi, Cibicides ventratumidus, Discoaster sublodoensis and Sphaenolithus furcatolithoides certifying the Middle Eocene age and developed in slope - basinal conditions; and secondly, medium - fine grained, silty carbonate to carbonate facies with Upper Eocene Globigerina corpulenta, Globigerinatheka index, Discoaster saipanensis and Chiasmolithus oarmaruensis characteristic for shelf - slope - basin domains.

During the Middle Eocene, a clastic input was derived by a high relief marginal delta through slope gullies and redeposited in a basinward successive progradation as a turbidite wedge, consisting of debris flow and high density turbidite current carbonate - siliciclastic deposits.

In the West Lebada area, during the Lower Paleogene, due to tectonic uplift generating major sea level fall, the Cretaceous platform margins retreated and subsequently scalloped margins truncated the shelf edge.The log response is typically blocky to locally serrated showing massive, amalgamated facies. Calcareous sandstones and subarkosic type present an excellent porosity, while the facies changes determine intrawedge permeability barriers. Seven distinct progradation stacking pattern geometries were recognized with repetitive facies changes from proximal coarse grained skeletal rudstone to distal thin fine grained, interbedded sandstones and skeletal packstones - wackestones. The Cretaceous shelf edge and slope fauna was reworked by highly energetic erosive influx.The basal Middle Eocene deposits exhibit an angular unconformity marked by Cretaceous chaotic reflections and Eocene offlap progradational terminations. Seismically, the Eocene formations may be considered a transition between basinward prograding strata and oblique tangential strata with a gradual basal dipping decrease, forming an upward concavity. This configuration reveals a high sediment supply and a constant sea level change, inducing a rapid wedge filling.

The Iris - Venus Area, during the Middle - Upper Eocene, a widespread oblique - sigmoidal offlap progradation took place as repetitive carbonate lowstand wedges and higstand systems tracts, the sea level change being accelerated due to rapid fault controlled subsidence. The log response reflects the transition between Middle Eocene medium - fine grained carbonate facies and Upper Eocene fine - grained carbonate facies deposited in basin - slope - shelf depositional systems. On the seismic lines, all the Eocene sequences are defined by high continuity and high amplitude facies. The transgressive systems tract is an exception, it grades from a seismic facies characterized by parallel high amplitude reflections to a mute seismic facies corresponding to a lithologically homogeneous sequence. In the other sequences, the toplap and downlap terminations delineate Eocene and Oligocene erosional and Eocene and Cretaceous depositional unconformities. The change from an oblique tangential facies to an oblique sigmoidal one shows a basinal energy lowering. The lowstand wedges are autochthonous and developed in restricted marine conditions being composed primarily of mud-rich carbonates represented by interbedded sandy skeletal packstone, porous spiculitic packstone and skeletal wackstone. The highstand deposits are composed of skeletal packstone - mudstone.

In East Lebada - Minerva area, the Upper Eocene lowstand wedge and highstand deposits are dominated by a series of carbonate couplets that downlap and onlap the lower - upper slope of the underlying Cretaceous deposits. Normal faults were reactivated influencing the deep subbasin configuration (Minerva). To the north, the subbasin is bordered by Cretaceous Heracleea Plateau that may be considered the line source for muddy carbonates. Local intraslope submarine/subaerial topographic highs have developed towards the south (East Lebada area). The facies consists of finegrained biogenic carbonate-thin siliceous ooze/muds and carbonate muds deposited as sediment waned in the deep water of starved slope and relatively close basin. The log patterns show the basal massive spiculitic packstone facies and the top serrated skeletal wackestone-packstone facies. The seismic reflection configuration displays a complex fill with an oblique - sigmoidal geometry in the lowstand wedge, a hummocky clinoform and a subparallel configuration in the highstand systems tract, and a parallel seismic configuration in the transgressive systems tract. The Middle - Upper Eocene deposits present low amplitude and low - good continuity, except for the last highstand systems tract characterized by moderate amplitude and continuity. There also occur slope slumps with chaotic reflections and high amplitudes. Topographic highs were subject to active subsequent diagenesis on muddy carbonate sediments, generating fracture systems with increased permeability, like in East Lebada gas field.

The Histria area sets in a ponded subbasin, controlled by splay faults, with local incised valleys and canyons systems. On the seismic lines, the carbonate turbidite complex has been recognized by dominantly subparallel, high amplitude reflections that downlap basinwards, with lateral changes towards slight convergent reflections of moderate amplitude. Low velocity Oligocene basinal shales (2100 - 2200 m/s) cover high velocity Upper Eocene carbonate turbidite complex (3800 - 4000 m/s). The turbidite body top provides a strong trough on the base. The variation of the turbidite complex thickness affects the seismic response quality along the basal reflection. The subbasin was deep, narrow, sediment starved, accumulating mainly shelf muddy decollement material, pelagic and hemipelagic sediments alternating with turbidite sediment influx. The Middle-Upper Eocene muddy carbonate shelf margin of Heracleea Plateau was the line source for the sediments that flowed onto the gentle deep gradient slope. Concurrently, intermittent submarine movements evolved as mud flow and turbidity currents through canyons in high-energy stages, alternating with pelagic and hemipelagic rain in quiet stages. The burial history diagram shows that the uppermost Eocene carbonate turbidite deposits are in the oil window, and the main phase of generation began 1.3 MA ago.

The intergrated analysis of the seismic lines and core data permitted to define the regional geological evolution: after the Paleocene regional uplift and subaerial erosion of the Cretaceous shelf - upper slope systems, locally siliciclastic influx was derived by fluviatile - deltaic systems during the Lower - Middle Eocene. The tectonic movements propagated in a southwest - northeast trend generating local district subbasins separated by tectonic highs that acted as local source areas. The subbasins present different dimensions: 4 - 38 km length, 4 - 16 km width and 204 - 1576 m thickness.

The main subbasin located in Iris-Venus area is deep, rhombic shaped, delimitated by normal faults, and may be connected with the strike-slip deformation component of Peceneaga - Camena Fault. The next subbasin, located on West Lebada area may be considered a cross- fault extensional zone. In the following subbasin East Lebada- Minerva area, normal faults generated a tectonic corridor characterized by high subsidence rate and local submarine/subaerial intraslope topographic high. The last subbasin localised in Histria area is a ponded one and captured highly organic matter rich muddy carbonate turbidites.

In all subbasins, there is a progression from structure control to sediment control deposition. There are carbonate- siliciclastic turbidite wedges and gullies fill (West Lebada), autochthonous carbonate wedges (Iris-Venus, East Lebada-Minerva), and muddy carbonate turbidites (Histria).

The carbonate - siliciclastic facies of turbidite wedge present reservoir quality with intercrystalline- intraparticle porosity and good permeability. The topographic high sediments were subjected to active diagenesis that enhanced local permeability. Part of hydrocarbon accumulations in Eocene reservoirs were generated by ponded carbonate turbidites which present over 0.5 TOC, in high geothermal gradient conditions.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado