The Bazhenov Suite: Unlocking the Potential

The Bazhenov Suite is now considered as a strategically important asset in terms of prospects for the Russian Oil industry. Rosneft is currently developing these deposits in the Salymskoye project (LLC “RN-Yuganskneftegaz”) and is planning its expansion.

In 2010, the company adopted the Innovative Targeted Program (ITP) – “The Development of technology for reclamation of carboneous argilliferous siliceous mass of Bazhenov Suite”. As part of this program, we are starting a series of publications on the Bazhenov formation. The first part of this series will review the geology of these deposits, and the second will examine the problems that arise in connection with their development.

Introduction
The Bazhenov Suite (BS) is one of the most studied, yet most unpredictable formations of petroleum geology in Russia. Suffice to say that it has been over 50 years since the Bazhenov Suite was first researched and over 10% of all petroleum geology papers have been dedicated to these formations. Research work is continuously underway and new publications and studies are issued, yet all this does not make this formation any more predictable.

Currently, over 70 deposits with industrial oil reserves are known to exist in the BS, but well drilling aimed at guaranteed production of oil even within the limits of contoured proven fields is still done by wild cat drilling. Oil deposits in BS are discovered by chance, there is no unified method of reserves calculation for Ju0 bed. That explains why the estimation of reserves sometimes varies by orders of magnitude ranging from 600 mln upto 30 bln. t. Operational experience at the Salymskoye deposit indicates of the absence of reliable technologies for profitable development of BS.

Geology of the Bazhenov Suite
The Bazhenov Suite is part of the layer with the same name. Bituminosity of the rock is considered to be the horizon’s distinctive feature. For the larger part of Western Siberia, the Bazhenov horizon, including Bazhenov Suite and, partly, Tutleim, Maryan, Danilov, Yanovstan and other formations, are represented with bituminous argillites [1].

Lateral analogues of the BS in the outskirts of the Suite are non-bituminous and low-bituminous rock of the respective parts of the Danilov (north-west), Yanovstan (north-east), Maryan (east and south) and other formations. BS rock itself is underlain with littoral and marine sediments of the Abalak and Georgievsk formations, which reflect the processes of the gradual marine incursion of Western Siberia during the late Jurassic period.

They are overlayed with sandy-argillaceous clinoform sediments of the lower Cretaceous. Accumulation of rock within the Bazhenov horizon corresponded to the conditions of maximal transgression of the sea Suite during late Jurassic – early Cretaceous period, when the water surface area was as large as 2 mln. sq. km.

Bituminous rock varies in age. To the west of the central BS sediments development, bituminous rock is gradually younger, from Tithonian to Hauterivian. This information shows the dynamics of Bazhenov Suite development, which should be considered when correlating bituminous rock sections and facies construction.

The BS is well traced by lateral and spreads over 1 mln sq. km in area, 10 to 60 m in thickness (average 30 m). In some cases, referred to as “anomalous sections”, thickness of Suite reaches 100 m and more. Deposits grow deeper from southern parts of West-Siberian plate to the northern. Formation roofs lie at 600 m minimum, 3800 m maximum.

The bituminous and organic-rich rock is often referred to as “black shale”, but relating to BS, “bituminous argillite” serves as the established term: it was initially used to outline the significant difference of BS rock from the host rock [2].

The lithology of BS is determined by ratio of biogenic and terrigenous constituents (fig. 1). Biogenic components are silica originated from organic skeletons and shells, kerogen which in some cases may take up the larger part of the rock mass; carbonaceous material may also bear rock-forming significance. Carbonaceous rock within BS may be of few different types. Primary biogenic carbonates are those remnants of pelecypodae, foraminifers, gastropods, teuthidae, coccolithophorae and pellet formations [3]. These could be organic formations of the late Jurassic and early Cretaceous age, which formed in shallow areas of the sea Suite which existed at that time. Carbonaceous rock could be secondary in relation to the primary biogenic siliceous rock. Biomorphic rock structure during carbonatization remains, but the silicate composition changes to carbonaceous. The secondary carbonates are products of chemogenic replacement. Fragmental material is mainly represented with argillaceous minerals, which were removed into the Suite from the adjacent land areas – the Ural plane on the west and the middle Siberian plane on the east, the Kazakh highlands on the south and the Altay-Sayan highlands on south-western edge of the plate (fig. 2). The remote location of the displacement sources from the central part of paleoSuite determined the income of terrigenous material into the central part of the Suite, primarily as part of argillaceous fraction.

Pyrite is also a stable component of the rock. With that, pyritization of a few stages is identified. Earlier generated pyrite is present in finely dispersed and forms tight organic mineral complexes with kerogen. Pyrite of later generation is unevenly developed and forms interlayers and lentils few centimeters in size.

In the general case, organic matter of BS, which in some interlayers reaches 60% of volume and more, has a primary nature and is related to vital activity of phytoplankton, algae organisms and surface vegetation. With that, marine sapropel matter (primarily skeletonless organisms – bacterial and algal) is most typical for the central areas of the Suite, whereas closer to its edges, share of surface organic matter increases. These conclusions are proved both by coal petrography and geochemical methods. Remainders of vitrinite are typical for outlying regions of BS sediments. The characterizing feature is increased organic content of the rock from the Suite base up towards its roof.

Along with high content of organic matter in the rock, increased concentration levels of many elements are also determined, such as Mo, U, V, Cu, Zn, Ni, As, Sb, Se, Ag, Au, Ba, Br. Their distribution along the section correlates with that of organic material. This increased concentration of microelements are normally explained by concentrating capacity of plankton life that had inhabited the ocean waters [4]. While describing the BB rock, the authors had found remnants of bivalve benthos organisms in many areas (fig. 3).

It is presumed that conditions of sedimentation in the Bazhenov Suite where characterized with sulphureous contamination of bottom waters. However, the first discoveries of burrow traces in high-carbon Bazhenov rock (Corg=10.5%) in the south-east of the plate near the Tomsk region signify that “we must concede the presence of areas with low oxygenation below the sedimentation surface – water in abyssal troughs at the bottom of Bazhenov sea” [5]. Discovered traces of burrow organisms, remnants of benthos fauna and various geochemical indicators show that at least periodically, near-bottom layers of the Bazhenov sea were free of sulphureous contamination. The authors believe that the Bazhenov sea was rather shallow. Its indication is the difference in deposit depths of undoform and fondoform parts of Achim clinocyclites (200-300 m), which filled the Suite later.

Oil bearing capacity
The history of combined development of Bazhenov and Abalak complex of BS deposits in the central part of Western Siberia proves its distinction from traditional reservoir development. Above all, the following features should be noted:
» Uneven distribution of wells with high initial flow rate across the area, and the rate may vary a significantly from a few tons per day to few hundred.
» Wells with oil inflow are characterized (though not always) by high temperatures and anomalously high formation pressure (AHFP), which may exceed hydrostatic pressure by 1.8 times. This signifies, firstly, of significant oil reserves which led to autofluidal fracturing of the reservoir and an increase in pressure, and, secondly, of a potentially high oil recovery factor (ORF).
» Significant increase in well flow rate after hydrofracturing (HF).
» Quite steep productivity decline: during the space of a year the flow rate may drop significantly. With that,  inflows of the main producing reservoir, CB1 (Carbonaceous bed), at Bazhenov-Abalak formation at the Salymskoye deposit may be sustained at 10 t/day level for a few decades.

According to the analyses of hydrodynamic research (HDR) on the wells of Salymskoye deposit conducted by S.G. Volpin and L.V. Zakrichniy (OJSC “VNIIneft”), the BB should be considered as stratum composed of reservoir intervals, which feed oil from the reservoir into the well and a matrix which feeds oil into these reservoir intervals. These conclusions were announced in the report “Determination of reservoir types in sediments of the Bazhenov Suite based on HDR data (Salymskoye deposit)” during the work seminar “Oil and gas bearing capacity of Bazhenov Suite sediments: problems and solutions” (CSTC OJSC “NC Rosneft”, Moscow, December 18, 2008). Based on the assessment of S.G. Volpin, oil-producing intervals account for 30% and matrix for 70% of oil production.

The main objective for the study of oil bearing capacity is obtaining information on the oil producing zones of the Suite. Having had no opportunity to study them from core sampling, the geologists had developed about ten models, which would explain the reservoir type and processes of its formation. Unfortunately, none of these have proven reliable so far. At the same time, analysis of formation mechanisms for BB rock capacity is impossible without the creation of a proper geological sediments model which would consider a variety of factors, the main ones being the processes of transformation of themineral and organic rock during the collection of sediments, diagenesis and katagenesis. Another complicating issue is the formation of fractures and colmation during technogenic intervention to the rock while drilling and raising core columns to the surface. The task of determining and correlating rock types is even further complicated by the fact that BB sections characterized by the core vary significantly even in neighboring wells. The sediments containing commercial oil deposits which have a “non-traditional” composition of reservoir rock, require specially developed methods of study. Such complex methodology does not exist at this time.

Oil producing zones of the BB have limited distribution across the area and are uneven across the section, their thickness being from the first few tens of centimeters to a few meters. Studying these intervals presents a problem due to the fact that fractured, foliated rock is practically impossible to extract as full core samples: they crumble and come out as sludge or rock fragments. Therefore studying these oil producing intervals from core samples is impossible, and their permeability and porosity properties (PPP) can only be assessed based on geophysical survey (GPHS) data. Moreover, permeability evaluation can only be done by hydrodynamic research (HDR) data.

The main issue of developing oil deposits in the BB is the low ORF. Currently, the oil recovery factor from the reservoirs of  theJu0 bed of Bazhenov Suite at Salymskoye deposit, calculated based on unobvious method is about 7%. Increasing oil recovery remains the main issue for the Bazhenov Suite. With that in mind, the primary interest lies in already formed oil deposits, which could be extracted using “traditional” methods. At a later stage, development focus could shift to mining the entire bed charged with oil during thermal destruction of kerogen.

Deliverability
Fractured cavernous carbonaceous sediments could be the primary producing intervals of Bazhenov-Abalak formation. The second type of reservoir could be fractured or foliated bazhenovites, formed primarily with kerogen and silica. The porosity of these beds, based on various evaluations, could reach 20% permeability exceeding 1 mkm2 (well open connected fractures). With that, porosity of the matrix measures in single percents (usually 1-2%), not exceeding 5% (one sample of 200 tested).

Having reviewed BB sections for the central and western part of its distribution, deliverability data, GPHR results and considering the enormous experience of earlier studies, the authors came to conclusion that most prospectives are carbonaceous interlayers, which can reach a few kilometers in length. At Bolshoi Salym they are represented by the CB1 bed, located on the border zone of the Abalak and Bazhenov Suite sediments. To the north of Bolshoi Salym, at the Surgut and Krasnoleninskiy highs, such beds can be found within BB stratum itself, and are identified as primary carbonaceous (algae, shelly hursts) or secondary-carbonated beds.

On this BB section, a few intervals may be identified where silicate rock underwent complete or partial carbonatization. The carbonaceous interlayer best traced across the area is the one confined to the limits of the upper and lower parts of the Suite, which differ greatly in density due to varying kerogen content.

Density features of these parts of the Suite allow for indication of fringes based on seismic data. Determination of carboneous bodies’ nature is a very important factor in forecasting their distribution.

Age and formation timing of organogenic carboneous formations may vary depending on the time of maximum transgression of the sea Suite. That’s why prospecting and distribution forecasting for carboneous rock must be done based on lithology-and-facies research. After conducting this research, patterns for alteration of lithological composition should be determined and assessed. In general, these patterns are apparent from an increased share of terrigenic (argillic) content in near-bottom and near-top parts of the bazhenov Suite and a rise of generational potential upwards the section.

Rocks with dominating siliceous and carboneous composition are potential reservoirs with fractured and porous-fractured capacity type. These are more predisposed to the formation of fractures due to tectonic movement or other influences expressed in a sudden decrease of pressure and a change of stress condition of this rock. Evidence of such influence can be seen on the core samples (fig. 4).

To evaluate the section properties in regards to which rock and at which initial conditions should undergo hydrofracturing, it’s necessary to characterize the section on stress-strain properties of its composing rock types, for which a study of rock in conditions of non-uniform compression is needed. The collection of core samples should include all major types of rock, with special attention paid to siliceous and carboneous variations. It is expected that the latter may be more fragile and fracturing their entirety will be possible with a lower external pressure. This information is necessary both for the design of HF and the evaluation of the formation in general.

Principal ways of approaching the development of Bazhenov Suite
In the presence of significant reserves the next key factor is reservoir permeability. At this time the main vehicle providing the inflow of fluid into the wells of Bazhenov Suite is the filtration of oil through a system of naturally extended fractures of the reservoir. However, the natural fracturing is poorly developed and permeability of the matrix is only about 0.001×10-3 mkm2. This is possibly the reason for lack of well migration with apparently oil-saturated core.

Considering the above, the main technological objective in developing the area is to create a secondary oil-saturated matrix through a dense system of induced fractures. This is done by drilling horizontal wells with multiple HF. Similar technology is widely and successfully used in the USA for shale gas mining from beds similar to those in the Bazhenov Suite. This technology has not been used in Russia. Thus the main objective of HF is inducing an intensive fracturing of the bed and creating secondary permeability within the well drainage area.

For the successful application of this technology and the determination of optimal HF design, precise assessment of geomechanical properties for the bed based on proper rock models is necessary.

Conclusion
Despite appearing well-studied, the Bazhenov Suite remains an unknown quantity for geologists as well as for operators. Modern studies are determined by the joined influence of  a number of factors, therefore the authors deem it necessary to define a range of important issues, for which clarification will be sought after further research work has been performed within the Targeted Innovative Project:
» reservoir modeling: material constitution, petrophysical properties and patterns;
» movable oil content: methods of determination and reserves estimation;
» technologies for the location of producing zones: patterns of distribution, capabilities of modern remote action methods and technologies of regional forecasting;
» mechanical-stress model: methods for determining parameters and modeling technologies;
» selection of the optimal development method: HF, chemical, thermal etc, and also pyrolysis.

List of Literature
1. Resolution of 6th interdepartmental stratigraphic conference on review and approval of revised stratigraphic diagrams for Mesozoic sediments of Western Siberia. – Bazhenov horizon of Western Siberia. Novosibirsk, 2003.
2. Braduchan Y.V., Gurary F.G., Zakharov V.A. Bazhenov horizon of Western Siberia. Novosibirsk. – M.: Nauka, 1986 – p. 216.
3. On the genesis of carbons within Bazhenov Suite of central and south-eastern regions of Western Siberian plate. / E.A. Predtechenskaya, L.A. Krol, F.G. Gurary [and others]. //Litosfera, 2006, #4. P. 131-148.
4. Zakharov V.A. Formation conditions of Volga-Berriasian high-carbon Bazhenov Suite in Western Siberia based on paleoecology data. From collected works “Evolution of biosphere and biodiversity”. – M.: Scientific publications partnership KMC, 2006. P. 552-568.
5. Zakharov V.A., Zanin Y.N., Zamiraylova A.G. First discovery of traces of vital activity in high-carbon black shales of Bazhenov Suite in Western Siberia//Geology and Geophysics, 1998. V. 39. – #3. – P. 402-405.

This article was published in the NK Rosneft Scientific and Technical Newsletter (Nauchno-tekhnicheskiy Vestnik OAO “NK “Rosneft”, No.4, 2010, pp. 20-25; ISSN 2074-2339) and won the 2-nd prize in the 2010 competition for the best publication in the newsletter. Printed with permission from the Editorial Board.

I.S. Afanasiev, PhD, E.V. Gavrilova, E.M. Birun (OJSC NC “Rosneft”), G.A. Kalmykov, PhD, N.S. Balushkina (Lomonosov MSU)