MALYSHEV N.A., BORODULIN A.A., OBMETKO V.V., BARINOVA E.M., IKHSANOV B.I.
Rosneft Oil Company, Russia

The Laptev Sea is a region with not proved hydrocarbon potential, but the Nordvikskoye and South Tigianskoye gas-oil fields, Olenekskoye and Khorbosuonsko-Kutyungdinskoe bitumen fields were discovered on its margins.

Oil and gas shows on the Laptev Sea margins are observed in a wide stratigraphic range – from the Riphean to Neogene.

The objective of the performed study was hydrocarbon potential appraisal and geologic risks assessment using the integrated basin modeling. Due to our conception on the Laptev Sea sedimentary cover structure and geological history of the region, we studied geologic risks and made an assessment of hydrocarbon potential for the Permian-Cenozoic formations. Characteristics of the main source rocks for this interval and the prediction of their distribution through the Laptev Sea shelf is described below.

The Permian Kozhevnichesky formations contain calcareous clays, with abundant organic matter (OM) in the Lenno-Anabarsky depression. These clays were deposited in lagoon and coastal-marine environment. Considering gradual basin deepening towards the Laptev Sea water area, wide development of the Permain source rocks is anticipated. Available tenor of organic carbon (TOC) show that OM of the Permian clays is mainly of sapropel type, containing from 0.55% to 3.7%, with insignificant plant material [2].

Maximum TOC content – up to 10% and more – was found in the Early Triassic formations on the Novosibirsk Islands [3]. OM is of sapropel type and characterized by very high HI values. Considering wide extension of Lower Triassic depressions on the basin margins and, most probably, on the Laptev Sea shelf, as well as high oil generation potential of Lower Triassic formations, we consider them as main source formations in the Laptev-sea basin.

The Lower-Middle Jurassic formations are characterized by higher OM (0.5-2 %) content in the Yenisei - Khatanga structural low [2]. The OM type depends on depositional environment — mainly sapropel in the shallow-shelf rocks and mixed humus-sapropel in continental and offshore formations. Accounting for dominating shallow-shelf, sometimes depression, facies distribution on the Laptev Sea shelf, determined from paleo reconstructions, the Lower-Middle Jurassic formations can have a significant impact on hydrocarbon presence in the section.

In the Upper Jurassic – Lower Cretaceous formations, OM content increases (up to 19.54 %), which is explained by abundant carbonaceous material, however, some shallow-marine formations of humus-sapropel OM are observed [2]. In the Upper Cretaceous section, it is expected to have mainly continental facies, and, despite of higher TOC (0.66-2.82%), OM is mainly of humus type, characterized by low HI values [2]. Therefore, Jurassic-Cretaceous formations are considered as mainly gas-source rocks.

The Cenozoic section contains OM-rich formations in the Eocene interval (azolic rocks). The content of TOC in diatomites, recovered from wells М0002 and М0004 on the Lomonosov Ridge, varies from 1 to 3 % [4]; OM is of a sapropel type. Due to rather high TOC values and 4-5 km Paleogene depths in depocenters, Eocene formations can have a significant impact on hydrocarbon presence in the section, particularly, because oil generation in cherty formations begins in PK3.

The history of the Laptev Sea sedimentary cover could be divided into three main stages — Permian-Early Cretaceous pericratonic period (in the western part), with rather low heat flow values and mainly quiet marine environment; Late Cretaceous-Early Miocene synrift period with active subsiding in depocenters, high heat flow values and sedimentation rate; Middle Miocene- Quaternary postrift period, with generally gradual decrease of heat flow values and sedimentation rates. The change from the pericratonic to synrift stage was accompanied by the Pre-Cenomanian pre-rift uplift with the amplitude of erosion up to 3 km, which was derived from paleo reconstructions.

Using the TemisSuite software package, 2D and 3D hydrocarbon system models were built to obtain a complex understanding of the region geological history, source rock maturity, conditions of HC generation, migration and accumulation on the Laptev Sea shelf. Due to absence of offshore wells and lack of model calibration, possible scenarios were evaluated by multivariant modeling using different characteristics of source rocks, heat flow and fault connectivity in time.

The lithological models were constructed for clastic-carbonate (Perimain-Jurassic) and clastic (Cretaceous - Cenozoic) complexes using paleo-geographic reconstruction, where we considered gradual grain size thinning going from continental to depression facies.

Heat flow values were taken from analogy with existing measurements onshore and in the northeast part of the Laptev Sea [1]. Calculations were carried using both constant (45.65 and 85 mWt/m2) and differentiated heat flow values (20 mWt/m2 for the pre-rift stage, 100 mWt/m2 for the active rifting and 65 mWt/m² for the post-rift time).

All models indicated the HC saturation of most parts of the geological section. Basin modeling shows that, depending on a heat flow magnitude, the Permian source rocks started to generate hydrocarbons in the Middle Triassic-Late Jurassic time, Early Triassic source rocks – in Jurassic-Later Cretaceous, the overlying source rocks – during the synrift stage (Late Cretaceous-Cenozoic period).

The Pre-Cenomanian uplift gave a significant negative impact on HC accumulation preservations — during the erosion and later due to absence of reliable seals, a significant destruction of HC accumulations and reaccumulation could take place. The strongest destructions are in the models with high heat flow values. However, the models with more probable heat flow differentiation showed that main source rocks started hydrocarbon generation after a heat flow growth and during the rifting under active subsidence that made it possible to predict preserved HC accumulations. The effect of other erosions was not modeled due to their low amplitudes.

The modeling with various sealing properties of faults proved HC presence in the crest of structures, even at constant high communication through faults from the Later Cretaceous till present time. This is related to the fact that maximum fault amplitudes are on slopes, so, typically, crests are less faulted, and uplifts have high amplitudes and big sizes. Therefore, this brings to a conclusion that the area has all conditions for HC generation and accumulation despite of active fault tectonics.

Following from 2D and 3D-modelings, the Permian-Early Cretaceous formations have completely realized their generation potential at present – in downwarping depocenters), or are in the main gas-generation zone (GGZ) – on uplifts. Hence, primary generation took place in OGZ. The Late Cretaceous-Cenozoic formations in downwarping depocenters are in OGZ, or at the beginning of GGZ, and mostly just realizing their generation potential.

The structural plan studies showed that the most traps were formed before active HC migration from source rocks, sometimes simultaneously, which is favorable for HC accumulation and preservation.

Based on 2D and 3D-modelings and the predicted area of source rock distribution, type and maturity, mainly oil presence is predicted in the Permian-Early Cretaceous formations and mainly gas presence in the Late Cretaceous-Cenozoic complexes.

Results of modeling and analysis of structural plan, sedimentary cover thickness, reservoir and source rock distribution, amplitude of the Pre-Cenomanian erosion on paleo-uplifts, presence of HC generation centers, migration paths and conditions for HC accumulation preservation, let us to divide the basin into prospective oil-and-gas zones. They are the West Laptev, Central Laptev, Anisinsky, and Omoloisky prospective oil-and-gas zones, continental slope prospective zone, and the East Laptev low-prospective zone.

The highest prospective is attributed to the West Laptev and Central Laptev oil-and-gas zones, located on the slopes of large HC generation centers and characterized by significant thickness of the sedimentary cover. The continental slope oil-and-gas zone and the Anisinsky, and Omoloisky oil-and-gas prospective zones are also of a significant interest for oil and gas accumulation prospecting, but require additional appraisal. The East Laptev zone is classified as low-prospective.

References
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Oil and gas of Arctic shelf 2008