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RAILEANU, A., I.S. POPESCU, L. STAN*, V. DRAGANUTA, Romanian Oil Corporation Petrom S.A., Geological Exploration Research and Design Center, Bucharest, Romania

Abstract: Albian Mixed Carbonate - Siliciclastic Reservoirs in Incised Valleys and Canyons from Central Moesian Platform, Romania

The studied area is located in Central Moesian Platform to the south of Pericarpathian Fault. It covers important Albian producing fields, mostly identified by classical structural methods.The Albian reservoirs randomly occur in subtle hydrocarbon traps that exibit various production levels.

This study is an attempt to reconstruct and define the depositional systems architecture and their reservoirs in terms of sequence stratigraphy, whose collector qualities were controlled by diagenesis and depositional setting.

The area is characterized by east-west and north-south trending faults. The former is parallel to Carpathian Orogene, with good continuity and large slip.They trapped Albian reservoirs and represent important migration pathway. The latter ones rest on the main faults and segment the oil fields. In addition, the north-south trending faults determined locally rapid uplift and erosion of source area or active subsidence.

On the regional seismic lines, the mapped Albian event occurs due to the acoustic impedance contrast resulted between the various carbonate-siliciclastic facies. This seismic event locally delineates depositional sequence boundary or lithofacies changes in different depositional systems. It also separates transgressive systems tract from lowstand or highstand systems tracts.

In order to define systems tracts, progradational, retrogradational and aggradational set patterns were separated on electric logs, which were grouped in depositional profiles (Figure 1).

The influence of diagenetic environments on pore system was estimated considering pore abundance, size, geometry, sorting and their connection type.

Distinct lithofacies associations of depositional systems were established based on integrated geophysical, biostratigraphical and sedimentological interpretation.

The following succession of the systems tracts was considered for Barremian, Aptian, Albian ages: (1) shelf margin wedge with carbonate facies developed in an autochthonous wedge (Lower Barremian to Upper Barremian-Aptian), (2) transgressive systems tract with carbonate facies (Lower-Middle Albian), (3) highstand systems tract with carbonate facies (Middle-Upper Albian), (4) lowstand systems tract with carbonate facies developed in incised valleys, canyons and prograding wedge (early Upper Albian ), (5) transgressive systems tract with carbonate-siliciclastic facies (late Upper Albian). Only the last three systems tracts were, here, detailed.

In the Middle-Albian, the highstand systems tract developed across the entire area, except for the eastern part.As the sea level slowly rose and fell, aggradational and progradational patterns succeeded, generating facies associations characteristic to the following marine domains: inner shelf, outer shelf, shelf edge and slope. Some of them exhibited shoals truncated by incipient channels. Major lithofacies is carbonate: skeletal wackestone-packstone, skeletal grainstone-packstone, peletal-skeletal packstone.

In the early Upper Albian, lowstand systems tract (Figure 2) developed proximally as incised valleys, canyons and prograding wedge. Due to rapid sea level fall, the area was subaerially exposed and high competence currents formed incised valleys into the previous shelf. Onto the slope, the valleys became submarine canyons, which revealed more shoals on the shelf edge. As the sea level rose slowly, a new carbonate platform evolved as a prograding wedge. The deposition took place in: (1) submarine channel system which segmented a shelf with low carbonate productivity, (2) shoals on inner shelf and shelf edge (Ciolanesti-14, Negreni-6, Glavacioc-11), (3) submarine canyons (Stefan cel Mare-5, Soparlesti-15, Preajba-13, Albeni-6, Popesti-9, Figure 2).The major lithofacies is carbonate, fine sandy carbonate on shoals and siliceous, sandy carbonate in channels.

In the first diagenetic stage, the subaerial exposure of shoals and valley/channel flanks underwent the effects of a freshwater vadose environment, which determined a high vuggy-intercrystalline porosity. In the second diagenetic stage, carbonate or siliceous sponge ooze filled locally the channels. Some of the pores enlarged because of active water circulation in a freshwater phreatic environment, which generated high intraparticle-moldic porosity.

In the late Upper Albian, maximum space accumulation and subsidence characterize the transgressive systems tract. Along north-south trend, the rapid flooding followed the subsidence axis direction, creating bay-like configuration. Depositional shoreline break gradually advanced landwards forming ravinement surfaces of reworked sediments in conjunction with longshore currents, storms and tidal action. These sediments were redeposited as offshore bars, parallel to shoreline. Frequently, these bars were deposited on the former shoal flanks or top channels. The main depositional systems include: shelf edge shoal, bay, channel and canyon. Major lithofacies changes to carbonate-glauconitite first and then argillaceous with thin intercalation of siltites or very fine sandstones. The offshore bars exhibit a high intercrystalline-vuggy porosity developed probably in a freshwater phreatic environment.

Defining of Albian systems tracts allowed evaluating reservoir potential in terms of depositional systems. It represents a basic premise in future exploration of subtle hydrocarbon traps in central Moesian Platform.

AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England