LUKIN, ALEXANDER, Institute of Geological Sciences of National Academy of Sciences of Ukraine

Abstract: Oil and Gas Potential of the Reef-Carbonate Reservoirs of Ukraine

Introduction

Biocarbonate reefs (BR) within Ukrainian petroliferous basins were established and explored far more later as compared with "classic" areas of their occurrences in North America, Western Europe and Russia. They are characterized by wide stratigraphic, geotectonic and paleogeographic distribution, biotic and geomorphologic diversity of local bodies, polygenetic nature of their reservoir properties. Therefore data of their study and exploration are of great interest for the comprehension of general regularities of reef formation.

Upper Silurian reefs of the Lviv depression

Tectonic development of the west and south-west parts of the East European Platform during Paleozoic was characterized by the formation of extended the Baltic- Near-Dniester -Trans-Dobrogea zone of pericratonic subsidence with intensive rifting and formation of several  pericratonic rift steps. Such tectono-geodynamic conditions favored the development of biogenic-carbonate build-ups and reefs during the epochs of carbonate sedimentation (Silurian, Late Devonian, Early Carboniferous). It would be recalled that the first fossil reef was revealed in Upper Silurian of Gothland Island in the Baltic Sea.

Carbonate builds-ups are established at several stratigraphic levels of Upper Silurian within the Lviv depression (Skalsky, Malinovetsky, and Bagovichsky horizons) [1].Their thickness varies from 15-30 to 60-75m. Comparatively small thickness combine with considerable width (5-8 km).They form lengthy zones extended for hundreds of kilometers. Barrier-reef nature of them is confirmed by existence of regular lithofacial zonation of the Upper Silurian deposits. The reef zones separated the shelf area from depressional clay-carbonate deposits (appearance of primary dolomites, sulphate-carbonate and sulphate rocks among shelf limestones). Facial zonation combines with typical paleogeomorphological features. The reef build-ups are asymmetric with steeper depressional slope (10-15°) as compared with behind-reef (lagoon) slope. Altitude of their crests varies from 10 to 20 m above the lagoon level.

Reef-forming biota is represented by stromatoporoids, corals, bryozoans, crinoids and algae. Core zones are formed by coral-stromatoporoid assemblages. They contain micritic domes side by side with framework builds-ups. Reservoir properties of the reef bodies are caused by fairly irregular manifestation of metasomatic dolomitization and paleokarst leaching. Two independent void systems induced by these factors are established. Secondary porosity connected with metasomatic dolomitization varies from 5-7 to 15-20%. This void system is saturated with oil here and there (Loklachi, Gorokhov, etc.). Paleokarst caverns and cave systems are water-bearing. Such peculiarities of reservoir properties like filtration and porosity make certain difficulties for prospecting, exploration and development of hydrocarbon (HC) pools. Therefore commercial petroleum potential of Silurian reefs within the Lviv depression is not established up to now in spite of oil and gas shows and bitumen impregnations. However, one ought to emphasize quite insufficient extent of their exploration. Promising prospect plays are expected to be bound with traps in draping over-reef structural forms. It has been confirmed by obtaining of commercial flow rates of natural gas from the from Lower Devonian (Lokachi)..Zones of the Silurian reefs can be traced from the Baltic Sea through the Lviv depression into the Dobrogea Foredeep.

Paleozoic reefs of the Dnieper-Donets depression and Dobrogea Foredeep

Stratigraphic levels of BR distribution here are related to Upper Devonian (Frasnian and Famennian), Mississippian (Tournaisian,Visean, Serpukhovian), Pennsylvanian (Lower Bashkirian, Upper Moskovian) and Lower Permian (Asselian). A number of them are timed to D-C transitional series. These passage beds are characterized by unique stratigraphic completeness within the Dnieper-Donets paleorift where their total thicknesses reach 2,5-3 km. That allows to retrace changes of reef-forming biotic assemblages and use them for more precise definition of D-C boundary position.Appearance of crinoid knoll reefs and banks in sub-zone C1ta2 (Rudenky strata) testifies to essential paleoecosystem transformations just at this level. Considerable biotic changes of BR allow specifying the position of C1t-C1v, C1-C2, C2-P1 boundaries.

Lower Carboniferous BR of the Dnieper-Donets paleorift and Dobrogean Foredeep are of special interest owing to their essential distinction from West European BR by facial diversity and widespread distribution of hydrocarbon reservoirs. Typical Waulsorthian facies are of limited distribution here while crinoid (crinoid-algal, bryozoan-crinoid, etc.) bioherms and brahiopod banks are widespread. Early Carboniferous carbonate facies in the Dnieper-Donets paleorift are characterized by wide distribution of crinoid calcarenites with great primary porosity occupied by heavy oils of early generations.

The Upper Mississippian (C1s) of the Southern slope of Voronezh Anteclise is characterized by presence of vast crinoid and coral biostromes while large-scale thick (up to 400-500 m) barrier reefs are localized within conjugation zone of it with the Donets basin (the Krasnoretsky faults zone) [2].

Mississippian — Pennsylvanian boundary in the Dnieper-Donets sections is marked by sharp change of reef-forming biota. Lower Bashkirian BR are created by algae Donezella-Hydractinian build-ups forming tremendous BR-complex (so-called Lower Bashkirian "slab") that stretches for 1500 km up to the Astrakhanian Arch where a unique gas-condensate field is associated with its carbonate reservoirs.

Early Permian reefs are formed by corals, algals, bryozoans, foraminifers, etc. The system of the Dnieper-Donets P1 saliferous depressions are fringed by BR zones too [3].

Upper Jurassic reefs

Malm belongs to the epochs to the epochs of the most intensive reef-carbonate development. Upper Jurassic reef-carbonate formations are characterized by extremely broad depositional range: from fresh-water and lagoon pelitomorphic limestones and marls to oceanic deep-water red-colored nodular limestones like ammonitico rosso. Late Jurassic reef-carbonate facies proper reveal a wide depositional-ecological diapason with extreme biotic diversity. Reef-constructing fossils are represented by corals, sponges, echinoderms, brachiopods, pelecypods (pectinites, oysters, rudists), gastropods, foraminifers in combination with different algae. Among typical late Jurassic build-ups one ought to mention various coral reefs, brachiopods and pelecypodal banks, foraminiferous-algal and algal-spongeous knolls, etc.Wide distribution of spongeous facies is characteristic of Upper Jurassic reef complexes. So silification is usual for carbonate reservoirs of that epoch.

All above-stated paleoecologic features are typical for late Jurassic of the Carpathian and Fore-Dodrogea — Crimea — Caucasus reefs belts. The Carpathian J3 reefs are localized within the Outer zone of the Carpathian Foredeep.Two stratigraphic levels of biogenic carbonate accumulation (the Rudkovskaya suite of Kimmeridgian lagoon sulphate-carbonate formation and the Oparskaya suite of Kimmeridgian-Tithonian marine carbonate formation) and two corresponding reef zones are established here.

The Rudkovskaya reef zone is mapped within the NW part of the Carpathian Foredeep within the band between Krakovetsky and Gorodoksky faults.Thickness of those reef massives varies here from 30-40 m to 130-150 m.They lie at the depth ranging from 500 to 1500 m.

The Oparskaya reef zone is situated between the Krakovetsky and Sudovo-Vishniansky faults. Thickness of separate massives within this reef cluster vary from 200-300 to 800-850 m.  The surface of the Oparsky barrier reef complex is intensively eroded, paleokarsted and overlain by Helvetian,Tortonian and Sarmatian. So presence of resorbed reefs and erosion remnants is a characteristic feature for late Jurassic reef belts in the Carpathians. Reef-carbonate reservoirs are screened here by Neogene gypsum, clay and marl seals. Presence of oil and gas (including commercial flow rates and numerous petroliferous manifestations) is established for some locations (Rudki, Sydovaja Vishnia, Medynychi, Grushev, etc.) and connected with upper parts of reef massifs.

Upper Jurassic (Oxfordian) reef zones are established within the Dobrogea Foredeep. Distribution of reef clusters is controlled by paleorift faults (Aluatsky and Tatarbunarsky grabens). The boundary between carbonate paleoshelf and depressional area (dark-colored siliceous clayey limestones, marls and black shales with total thickness of Oxfordian more than 500 m) coincides with the Chadyr-Saratsky tectonic suture. According to all available data extensive (over 150 km) and wide (up to 25-30 km) barrier-reef belt is situated here. Carbonate reservoirs are represented by bioconstructed, bioclastic and oolitic limestones (the degree of dolomitization varies in a great range).They are screened by clays, marls and hydrated anhydrocks (with different ratio of anhydrite and gypsum) of Kimmeridgian-Tithonian. Oil and gas manifestations are established in a number of prospecting wells (Ferapontievskaya, Andrushinskaya,Tarutinskaya, Baymaklinskaya and other areas). These data together with presence of the J3 of Kerch Peninsula allow to mark out presumably petroliferous character of Upper Jurassic reef-carbonate reservoirs zone across the Black Sea.

Micropetrographic and isotope-geochemical data testify to heterogeneous complicated nature of carbonate reservoirs related to the J3 reef (paleokarst percolation, hydrothermal leaching, metasomatic dolomitization with resorption of limy skeletal remnants, diverse tectonic and exogenic fracturing with insignificant role of primary porosity).

Upper Cretaceous reefs

A great carbonate-forming epoch of late Cretaceous was characterized by substantial changes of the paleogeographic, paleoecologic and paleogeodynamic conditions of reefs formation. Conditions of intensive biogenic carbonate accumulation (chalk and chalky rocks) upon vast areas were not favorable for the reef formation.The latter were concentrated mainly within equatorial belt of the Tethys (sensu lato) and Caribbean Basin. The set of reef-constructing biota and lithogeodynamic style of reef formation was connected with vital activity of rudists (rather common, but far from always together with corals). Stratigraphic distribution of these pelecypods is restricted by interval J3-K2, but their proper reef-forming role began in Albian and came to the end of Maastrichtian. Just rudistous banks are main units of core parts of the K2 reefs. In contrast to the Late Jurassic ones the K2 reef complexes are characterized by dominating of carbonate platforms and mega-atolls over the linear reef zones. The signs of their presence are recorded within the Black Sea equatorial area. Carbonate platforms are mapped in the Cenomanian — Coniacian — Turonian, Santonian, Campanian, Maastrichtian, and Danian-Montian complexes within the area of the NW shelf. It is not excluded that these platforms are framed by barrier reefs and are mega-atoll-platforms in fact. In spite of insufficient extent of lithological and paleoecological studies of above mentioned carbonate complexes one ought to note the certain similarity between the facial zonation of the Gulf of Mexico (Albian — Cenomanian) and the Black Sea (Upper Cretaceous). This resemblance would increase if one takes into consideration the data of wide-distributed dolomitized bioconstructed and bioclastic limestones (in spatial proximity with tuffs and carbonate-volcanic breccia) in southern parts of the Black Sea.

Conclusions

Morphological variety of local reef-carbonate bodies (knoll and patch reefs, atolls and faros, barrier and fringe reefs, etc.) and their structures (organic framework reefs, banks, bioherms, mud mounds, calcarenite accumulations, etc.) are connected with the diversity of reef-forming ecological, paleogeographic and lithogeodynamic factors [4]. Petrophysic, mineralogic and isotope-geochemical data testify to multiformity of reservoir-forming factors (paleo-karstification, various stages of metasomatic dolomitization, carbonate resorption caused by microbiological oxidation of hydrocarbons, and so on). Combination of all above mentioned factors stipulate great morphogenetic diversity of hydrocarbon traps and pools. The largest Ukrainian oilfield, the Lelyakovskoye one, is confined to early Permian dolomitizated ecological reef. A number of gas-condensate and oil pools are controlled in the Dnieper-Donets basin by diverse Mississippian BR bodies (Machekhskoye, Boghatoyskoye, Kampanskoye, Seliukhovskoye, Muratovskoye and other fields). The studies carried out attest considerable perspectives of diverse morphogenetic types of carbonate reservoirs and traps within reef-carbonate complexes of different age in Ukraine.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England