Rosneft Basin Modeling to Assess the Hydrocarbon Potential of Offshore Areas
N.A. Malyshev, V.V. Obmetko (Rosneft Oil Company OJSC, RF, Moscow),
A.A. Borodulin (RN-SakhalinNIPImorneft LLC, RF, Yuzhno-Sakhalinsk)
The depletion of the resource base in Russia’s historical oil producing regions had brought about the necessity of developing remote areas in the Far North, Eastern Siberia and offshore regions. Performing geological exploration works (GEW) in these regions entails technological challenges and high financial expenditures. In this context, the risks of drilling nonproductive wells have to be minimized.
Modeling of hydrocarbon systems formation is applied at Rosneft Oil Company OJSC since 2004 to minimize geological risks during area selection, planning and geological exploration works. The modeling is performed using Beicip Franlab (BF) – TemisSuite, LOCAS/CERES, Dionisos and Qubes software products in two phases: 1) at the regional stage – for evaluation of oil and gas bearing potential of poorly explored sedimentary basins; 2) at the prospecting stage- for selection of primary assets and forecasting formation pressure in accumulations. A total of 19 projects have been performed since 2004 in various regions of Russia with creation of over fifty 2D and 3D models (fig. 1).
Fig. 1. Regions and areas for which Rosneft performed 2D and 3D modeling of hydrocarbon systems formation: 1 – “Grey zone”; 2 – Admiralteyskiy swell; 3 – Pechora sea; 4 – Chernyshov ridge; 5 – South Kara basin; 6 – Vancor field; 7 – Yenisei-Khatanga trough; 8 – Laptev sea; 9 – Chukchi sea; 10 – Lisyansk, Kashevarovsk and Magadan basins; 11 – Shelikhov gulf; 12 – West Kamchatka shelf; 13 – Astrakhan, West Schmidt areas; 14 – East Schmidt, Kaygan-Vasyukan, Veninsk areas; 15 – Tatar Strait; 16 – Black sea; 17 – West Kuban trough; 18 – North Kaspian; 19—Turkmen shelf
Technical Approach
The most important issue for correct forecasting of oil and gas bearing prospects is completeness of geological data in the model for various range objects (from basin to accumulation), including evaluation of development of source rock intervals (SRI), reservoirs and impermeable seals in section and areally, thermal history of the region, presence and permeability of faults, extent and duration of perturbations, erosions etc. A full set of such data is available for assets in prospecting stage of GEW at best. At the regional stage of the research during the preparation of initial modeling information, a complex analysis of all available geological, geophysical and geochemical data is performed. This is especially important for frontier sedimentary basins of Russia’s shelf regions.
Complex approach to study of sedimentary basins includes:
» collection and analysis of accumulated geological and geophysical data, including well drilling results, descriptions of natural outcrops, seismic and gravity-magnetic data, laboratory analytical results;
» creation of a single seismic project, interpretation of new data or re-interpretation of earlier seismic data;
» creation of a structural model and tectonic zoning of the region;
» reconstruction of regional history and sedimentation conditions using seismic facies (sequence stratigraphic) analysis.
This complex data is then used for modeling the formation of hydrocarbon systems, performance of oil and gas geological zoning, estimation of resources and geological risks, ranging the prospective assets and for preparation of recommendations for further directions of GEW [1].
Up to the present time Rosneft has collected a significant volume of seismic data for Russian sea shelves (over 250 thousand km), as well as well data (over 20 wells). Also, numerous reports and scientific publications where used for basin analysis.
Re-interpretation of seismic data in Open Works (Landmark) and Kingdom software has contributed some significant adjustments and in a number of cases, allowed to create principally new structural tectonic models for formation and morphology of sedimentary basins in Russian Arctic shelf [2, 3].
History reconstruction for any given region is done with consideration of all accumulated geological information: lithology facial composition of sediments; earlier performed paleogeographic reconstructions; type of scale of nondepositional hiatus detected in section and etc. Stratification of reflecting horizons and actualization of paleogeographic constructions is performed based on this data with consideration of adjacent onshore well data. Analysis of the source area allows possible forecasting of the reservoirs distribution, composition and properties. In addition to this, seismic facies and sequence stratigraphic analysis are performed for the estimation of the reservoir and the impermeable seals distribution in the plan and in the section based on the seismic data. This is the first time such an approach is used for a number of regions (Laptev sea shelf and the Russian part of the Chukchi sea shelf).
Oil Company Rosneft OJSC created and is constantly updating a geochemical database with a purpose of forecasting the development and composition of SRI in the sedimentary basins of the Arctic shelf. Considering these materials along with the paleogeographic and lithology facies reconstructions of the region, the distribution features of source rock are evaluated.
Modeling Hydrocarbon System Formation
The modeling of hydrocarbon systems formation itself is done primarily in the TemisSuite software. The cycle consists of 1D, 2D and 3D modeling. Data preparation includes the creation of structural models, the reconstruction of eroded formation thicknesses, forecasting of reservoir and impermeable seals distribution and the source rock in the section and area. The modeling process includes calibration of the thermal model, assessment of how the most critical parameters (scale of erosion processes and faulting) influence the formation and preservation of formations and forecasting of the reservoir saturation with carbohydrates (fig. 2).
The scarcely available data for the sedimentary basins of the shelf creates a number of problems mainly related to the numerous uncertainties. This is why the authors perform multi-version modeling of the hydrocarbon accumulations formation and evaluate the model’s sensitivity to changes of certain parameters. Usually, pessimistic, optimistic and most probable scenarios are assessed. Without a doubt, most uncertainties may only be resolved after drilling appraisal or prospecting wells.
Let us review some of the problems and approaches to their resolution showcasing various basins of the shelf.
Stratification and completeness of the section. Ambiguities of the stratification and the completeness of the sedimentary section is typical for the least explored sedimentary basins in The Eastern Arctic [3-10]. The reason for this is the absence of deep wells on the shelf and the impossibility of definite reflection identification to the wells drilled in the American sector of Chukchi sea and along Laptev sea coast.
For the western part of the Laptev sea shelf, where the views on the stratigraphic composition of the sedimentary section disagree the most, modeling was performed both for the version of the Permian-Cenozoic scenario and for the version with smaller stratigraphic intervals, in volumes including only the Apt-Cenozoic formations. For the first version, the model revealed that the edges and most elevated structures of the region feature significantly higher hydrocarbon content. The structures that were primarily saturated with oil were identified. The results of the second version emphasize the maximum hydrocarbon content for most of the immersed traps in the basin and the gas composition of fluids filling these traps.
Thermal history. The modeling, considering all the various thermal histories is conducted for all the basins under review. With that, both modern heat flow measurements (as a rule they are sporadic) on the surface and the formation temperatures are based on well data as well as the principal indicators of the paleotemperatures – vitrinite reflectance values (Ro) for both core and natural outcrops. The analysis of the region’s thermal activity periods related to rifting and magmatism is also important. Usually, different options are analyzed either at constant heat flow or heat flow differentiated in time. Modeling for Laptev sea shelf revealed that changes to the heat flow have a significant impact on the time when hydrocarbons began to generate and migrate, as well as the extent to which the traps were filled with hydrocarbons and on phase composition of fluids in forecasted formations. Resolving thermal history related uncertainties is possible by calibrating the models based on drilling results, increasing the number of modern heat flow measurements and more precise reconstruction of region’s thermal activity phases.
Analysis of source rock intervals. The next important component for modeling is consideration of the SRI structure peculiarities and their geochemical characteristics. In regards to the least explored sedimentary basins of the eastern Arctic and north of the Sea of Okhotsk, various researches have significantly different positions on the assessment of development of various oil and gas bearing rock potential in the section. The authors are studying the SRI in well section and in outcrops on islands and in coastal onshore areas as well as attract the data for the nearest analogous sedimentary basins. Ambiguity of initial data on the position in sections and the SRI characteristics explains the necessity for multi-version modeling with consideration of various source rock characteristics. For the shelf areas where siliceous SRI are developed in sections, modeling is also done with consideration of various kinetic characteristics of kerogen. With that, modeling shows ambiguous results. For example, various kinematic kerogen characteristics have an insignificant impact on the type of traps and on the extent to which they are filled for the Sea of Okhotsk aquatic area, because siliceous source rock here have just entered the oil generation phase and have not impacted significantly the phase composition of hydrocarbons and the filling of traps here. At the same time, in the South Kara basin these factors are manifested to a greater extent since siliceous SRI here have primarily completed their generation potential. Altering such factors as SRI thickness, content and type of organic matter has the largest impact on the saturation of section with hydrocarbons.
Evaluation of impacts from nondepositional hiatus and the extent of rock erosion. Modeling with consideration of various extents of rock erosion revealed that eroded sections below 300-500 m amplitude as well as large erosions which happened before the main stage of hydrocarbon generation began have insignificant impact to the formation and preservation of accumulations. Thus, significant by its scale pre-Jurassic rock erosion in the North Chukchi basin had an overall negative impact to hydrocarbon accumulations formation in overlying units due to the erosion of Shublik, one of the principal Triassic source rock formations for the section. In section below the erosion surface, pre-Jurassic processes had a positive impact, contributing to formation of stratigraphic type traps. The main hydrocarbon generation and migration stage here took place at a later time. Similar situations are noted for sedimentary basins in the northern part of the Sea of Okhotsk.
Large hiatuses and erosions that happened here either after or during the main phase of oil and gas generation and migration from the source to the traps had a negative impact of formation and preservation of hydrocarbon accumulations. Thus, modeling revealed an almost entire destruction of hydrocarbon accumulations in near-edge zones of uplifts, along the Wrangel-Herald oblique slip faults zone in North Chukchi trough during early Cretaceous and early Cenozoic erosions.
Evaluation of faulting impacts. Experience of modeling with consideration of various conductivity of faults shows that at the regional phase, in absence of data on conductive role of faults given large uplifts and not very significant vertical amplitude of faults, no significant differences are found in regards to saturation of attic zones of structures. Modeling the impacts of fault tectonics at this stage may be neglected as it only introduces more uncertainties into the model. The fault tectonics factor is expedient in modeling prospecting areas, when it is possible to use analogue fields benchmarking to assess conductivity of faults varying in scale, type and direction.
Overall, based on 2D modeling experience for hydrocarbon systems formation at Rosneft, usually 12 to 20 versions of the model are calculated. Final conclusions about prospective capacity of assets and their ranging are based on correlation of favorable and unfavorable scenarios of hydrocarbon accumulation formation processes.
Three-dimensional modeling is typically done for the most probable scenario. It is aimed at determination of maturity of principal source rock in research area, selection of hydrocarbon generation centers, estimations on volume of generated and migrated hydrocarbons, detection of drainage zones for prospective assets and evaluation of phase composition of fluids in the accumulations (fig. 3). In presence of statistical regional data on the scale of hydrocarbon losses during migration, 3D modeling is used for quantitative resources evaluation for possible formations.
Results of basin modeling application. The first results of basin analysis application at Oil Company Rosneft OJSC were obtained for the Vankor region in 2004. (N.A. Malyshev et all, 2004).
In 2006, LOCAS software was used to model the formation and infilling with hydrocarbons for the structurally complex Vorgamusyur asset at thew Chernyshov ridge in the Timano-Pechora basin (fig. 4). Based on the modeling results, favorable forecasting for hydrocarbon saturation of the lower Permian and Carboniferous formations in underthrust, part of the Vorgamusyur structure, was obtained. Drilling of well 2 Vorgamusyurskaya confirmed the forecast. Lower Permian carbonates manifested an oil inflow of 2 m3/day. Due to poor reservoir properties of lower Permian carbonates and insignificant reserves, this formation was declared non-commercial.
In 2007-2012 based on the results of complex research on the oil and gas bearing prospects for Russian sea shelves using basin modeling technology, hydrocarbon resources were re-estimated, prospective assets were ranged and recommendations were prepared on licensing and further GEW directions for the shelves of the Pechora, Barents, Kara, Laptev, Chukchi, Caspian, Black Seas and Sea of Okhotsk.
References
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The article was published in the “Oil Enterprise” magazine (“Neftyanoye Khozyaystvo”), No.11, 2012, pp.14-17. Printed with permission from the Editorial Board.
