Gazprom Neft: Achimov Deposit – Key Development Issues from Different Study Perspectives

INTRODUCTION

The Achimov horizons are deep-water sandy and clay strata’s associated with the fondoform part of the Neocomian cyclic wedges [1]. Reserve recovery in these deposits is still less than 10 percent, with a mere handful of license projects in which the Achimov horizons have entered the commercial production phase despite dozens of years of research on the Achimov’s geology. This is due to the complicated geological architecture of the horizons, their vertical and lateral heterogeneity, low porosity and permeability, and high water saturation.

The Achimov formation features prominently in the Gazprom Neft resource portfolio development strategy because of the enormous resources lying there (Figure 1).

In view of its complicated geological architecture and distinct development, it seems justified to single out the Achimov formation as a separate area of study. To improve knowledge and experience, define pressing challenges, develop new technologies, and promote the best solutions, the company has created an Expertise Integration Centre for the Achimov formation.

This article summarizes the geology and technologies related to exploring, characterizing, and bringing the Achimov deposits into production.

HISTORICAL OVERVIEW

The Lower Cretaceous deposits were first characterized and grouped into the Achimov sequence in the section of the south-eastern areas of the West Siberian Province by F. Gurari.

Much later, A. Naumov showed the clinoformal configuration of the West Siberian Neocomian that eventually became mainstream among geologists.Today, there is not much debate about the clinoformal architecture of the Neocomian and splitting the deposits in the Achimov formation into relatively separate stratigraphic intervals. The occurrence of the Achimov formation as a single flat wedge is considered to have been demonstrated, along with two regions of clay alteration: the eastern region resulting from facies, transitioning into a silty clay clinothem, and the western region, which can be attributed to a distal thinning of sand beds in the depositional area far from the terrigenous source.

Multiple paleontological and specific analyses performed on cores established with a high degree of certainty that the timing of the Achimov formation corresponds to the stratigraphic series between the Berriasian in the east and the Lower Hauterivian in the west. This period is marked by the prevailing deep-water depositional environment in the West Siberian Neocomian paleobasin.

UNCERTAINTY FROM A REGIONAL PERSPECTIVE

According to the lithologic and paleogeographic criteria and morphological features of the seismic reflections, the depositional wedges may be regionally divided into three sections: eastern (shallow-water), middle (deeper-water), and western (the eastern dipping clinoforms) [3].

The eastern area of the Achimov formation is distinguished by the following structural clinoform features:

• Small clinoform thickness and depth (∆t = 80 to 90 msec) suggesting a shallow water basin

• Indistinct boundaries between the undaform, clinoform, and fondoform parts of the section preventing lithologic and facies zoning

• Indistinct change from shelf sandstone to argillaceous slope deposits and then to the Achimov sandy silt structures

• No apparent sigmoid reflections (the single wedge seismic facies) in the seismic sections

• High net-to-gross lithologies: small share of argillaceous sequences inconsistent in size, only forming seals locally

Proven reserves and estimated potential of the eastern area are relatively small [2].

In contrast with the eastern area, the middle area is distinguished by the following lithologic and facies features in the Achimov formation section:

• The clinoforms identified within the area overlapping each other nearly throughout

• More apparent sigmoid shape of reflections progressively steeper to west

• Abundant structures specific for turbidity features, fractured rock areas, and widespread mixed porous and fractured rock reservoirs

• Argillaceous silt slopes and clinoform depth (∆t = 280 to 310 msec) progressively steeper to the west, suggesting a deeper sedimentary basin

• Satisfactory (in the east) and distinct (in the middle and west) boundaries between the undaform, clinoform, and fondoform parts of the units

Hydraulically and dynamically isolated reservoirs, consistent cap and reservoir rock, and multiple types of the sand bed overlaying seals make the middle area of the Neocomian clinoforms most promising as an oil and gas bearing stratum with the most discovered hydrocarbon deposits.

The east dipping clinoforms of mainly Hauterivian-Barremian age occurring in the western section of the basin demonstrate the following features:

• Arching structure with no well-developed shelf, suggesting a lack of sand within the units in question

• Almost complete lack of sandstone and widespread clay with thin siltstone seams.

No hydrocarbon deposits have been detected within the western region. The oil and gas potential of the Neocomian is estimated to be extremely low [3].

Another spatial trend pattern observed regionally is from south to north. The primary reason for it is the Achimov deposit thickness increasing northward, which is connected to a relative subsidence of the West Siberian Basin and high rates of sand input. The oil and gas saturation of the northern part of the West Siberia is unparalleled in its distinctive structural history for the plate area in question, rather than an extensive deposition of sand bodies.

The endogenous factors associated with rifting affected the extent of oil formation and almost complete saturation of the lens shaped, hydraulically and dynamically isolated reservoirs deposited at the bottom of the Neocomian section. These processes were accompanied by emerging abnormal formation pressure. In summary, the Yamal Nenets Autonomous District and Kara Sea are an anomalous dynamic fluid system and the oil and gas accumulation zones with the low-porosity and low-permeability reservoirs hold impressive hydrocarbon potential. Their deposits have an overlaying seal, no reliably identified fluid contacts, and a lack of elevationally differentiated oil and gas bearing areas.

The Khanty-Mansi Autonomous Area and the southern Tumen Region are hydrostatic pressure areas with sheet-like, uplifted oil deposits with sealing lithologies and structural barriers.

The current regional zoning model for the Achimov formation needs to accommodate the available geological and geophysical information. An improved clinoform zone distribution concept for the company’s operation area will lead to a better understanding of regional environment and rock volume and composition in the Achimov formation both inside the license area margins and high potential unallocated portion.

Designations changed with new information emerging about the Achimov bed structure. N. Nesterov proposed the prefix “Ach” that was approved for indicating beds in the government hydrocarbon reserve register. Meanwhile, depending on the quantity of beds identified in the section and their corresponding hydrocarbon deposits, each field used its own designations. Accordingly, some challenges surfaced: beds with the same designation at the nearby fields correspond to different stratigraphic datums; beds with different designations in the government register belong to the same deposits, and new exploration discoveries have unidentifiable designations [4].

In the author’s view, the most reasonable designation for the Achimov formation recognizes relations between the undathem, clinothem, and fondothem sediments comprising a single transgressive-regressive sedimentation cycle. A multidimensional architecture analysis of the Achimov and synchronous shelf sediments establishing connections between the two is crucial.

As an example of such designation, Figure 2 shows a Neocomian section for the northern Pre-Ob.

Designation is one of the priorities. Solutions to this type of issues have both a fundamental and a specific instrumental value for predicting new oil and gas bearing areas, areas with better reservoir properties, hydrocarbon traps, and selecting analogs.

Fluid phase zoning is also uncertain. Insufficient geochemistry and a rather complicated distribution of oil and gas bearing properties in the section gave rise to a series of hypotheses about formation of deposits in the Achimov formation. According to one of them deposits may have been formed through upward hydrocarbon migration from the lower Upper and Middle Jurassic sediments or, according to another one, through their own generation and accumulation potential.

The study of deposit distribution suggests that the southern areas are mainly oil bearing and the northern and middle areas of the West Siberia are mixed oil and gas bearing. In a similar vein, oil deposits at a depth of 4 km call for an explanation of their formation. According to the D. Soin and V. Skorobogatov research, the catagenesis stage for the Achimov formation rock nearly entirely falls within the MK1-MK3 gradation range corresponding to the “oil window” [5]. High clay content and a rather limited sand laminae occurrence led to a small hydrocarbon migration from the Achimov formation. The secondary migration flows were also restricted to an inner reservoir inside the individual lens horizons without any large lateral, cross reservoir hydrocarbon communication.

Another pressing issue is an impact analysis for fault tectonics in the Recent time (the Neogene) and oil bearing capacity and deposit quality. Despite the faults with varying separation identified or predicted in many West Siberia fields, their effect on re-depositing the reserves in the higher horizons may be assessed with a high degree of certainty for only some fields.

UNCERTAIN FEATURES

Their complicated geological architecture is seen as a factor for the complexity and poor predictability of reservoirs evolution history in the Achimov deposits. They are connected with a deep-water environment, gravity mechanism, and irregular deposition.

One major uncertainty indicative of the complicated architecture of the Achimov formation is a vertical and lateral reservoir distribution. The deposits have low vertical and lateral reservoir connectivity related to their associated facies (a distal/proximal fan end, fan apex, or supply channel). Further, there is a technical issue of identifying the clay alteration margins.

Obviously, a true prediction of the reservoirs distribution in the Achimov formation is only achieved by integrating the 3D seismic and well logs. But even with the seismic, the lack of a single integrated data set, that does not support the quantifying reservoir distribution, adds to the complexity. The small thickness of the distal fan is significantly below the capability of the instrument’s resolution.

Choosing an appropriate method to evaluate the rock’s porosity and permeability for the interval causes some petrophysics challenges since there is a lack of understanding of the best combination of logging and core sample data. The fine lamination in the Achimov deposits impedes distinguishing between oil and gas reservoirs, making it necessary to resort to state-of-the-art well logging tools with a high vertical resolution. A highly directional rock porosity and permeability is detrimental to the accuracy of the petrophysical models for porosity, permeability, and water saturation. The lens-shaped deposit architecture and hydraulically and dynamically unconnected laminae hinders the application of the capillary models anchored in defining water saturation profile depending on the height above the free water level.

In addition to the reservoir properties and distribution, speaking of uncertainty, from the feature perspective, equal weight should be given to the Achimov deposit saturation. Study of the technology applied to exploration and development of the studied unit shows a hydrocarbon and water influx mixed at a varying ratio nearly throughout. Together with core samples, it suggests there are undersaturated reservoirs in the Achimov formation (with no water saturation below the critical Sw) in the majority of the West Siberian Basin. Hence, the oil only areas are not typical for the deposits in the studied interval, and production has a high water cut.

The issue of the water-oil contact (WOC) depth for the discovered fields is still open: the Achimov formation inherently has a locally varying porosity and permeability and the WOC can fluctuate widely within one field; the low thickness, hydraulically and dynamically unconnected lenses present formidable challenges for a better understanding and evaluation of inflow; determining the WOC by well logging is difficult; laminated and dispersed clay volume leads to an underestimation in rock resistivity and, consequently, to an inaccurate oil saturation factor.

Besides geological uncertainty, from the feature perspective, due weight should be given to the Achimov deposit development. The most usual challenges are a high initial water cut, substantial pressure decline rates, and inefficient reservoir pressure maintenance (RPM) system.

Initial water saturation for the company projects varies from 20 to 90 (with an average of 60 to 70). The primary reasons behind it are:

• Isolated water saturated laminae in the Achimov deposit section

• Hydraulic fracturing crack propagation to the higher water saturated deposits

• Mobilization of reservoir water trapped inside the unconnected pores prior to hydraulic fracturing

• Entrapment of residual oil

• Water breakthrough from injection wells at closing of hydraulic fracturing cracks

High reservoir pressure decline rates (exceeding 70% for the first year) are caused by a low-efficiency RPM system: in most cases, production wells do not respond to injection. Some injection wells also show a substantial decline in injectivity in the first months of operation that can be connected to the injection system properties. 
For low-permeability reservoirs, injection efficiency is largely dependent on degree of particulate matter and emulsion removal from water and a choice of a perfect fluid salinity and composition to control reservoir fluid and rock reaction [6]. Cost- effectiveness of introducing an RPM system in the Achimov beds remains an issue of concern. One of the options to increase oil recovery factor for low-permeability reservoirs is to use gas as a displacing medium.

Uncertainty from the feature perspective has a profound effect on the development efficiency and, accordingly, the  projects success; however, the nature of local uncertainty is, to a large extent, rooted in the regional attributes. It is interesting to note that, starting from a feature level down, the technical uncertainty matches geological uncertainty.

CONCLUSION

In light of a challenge and uncertainty review for the Achimov deposits, a few key areas are pinpointed for further development.

Regionally, major investigation should be aimed at classifying West Siberia by the Achimov deposit potential and identifying the exploration priority areas. To this end, the following exercises should be conducted:

• Identifying regional distribution patterns for the Achimov formation (variation in porosity and permeability, fluid phases, net-to-gross, and bed thickness) from the
company’s available information to improve understanding of environment and rock volume and composition in the Achimov formation

• Improving bed designations and margins of the regional cyclic wedges for predicting new oil and gas bearing areas, areas with better reservoir properties, hydrocarbon traps, and selecting analogs

• Expanding well logs, performing specific and biomarker analyses on core and reservoir fluids to refine the source bed model and migration scenarios further and predict the oil and gas accumulation zones

Consequently, the following areas are identified for further development and research:

• Developing new approaches to seismics processing and interpretation in order to map prospective targets within the clinoform body in detail

• Adjusting current techniques or creating new ones to analyze the Achimov deposit core samples to determine relative permeability, fluid content, and porosity and permeability as accurately as practical

• Developing a technique for well logging and outcome interpretation to clearly describe a highly dissected, laminated section

• Preparing a multidimensional program for research and most profitable development of the Achimov formation by Gazprom Neft supported by pilot project outcomes for different areas.

REFERENCE

1. Gurari F.G., Stroenie i usloviya obrazovaniya klinoform neokomskikh otlozheniy Zapadno-Sibirskoy plity (Istoriya stanovleniya predstavleniy) (The structure and conditions of formation the clinoforms of Neocomian deposits of the West Siberian Plain (History of representations the formation)), Novosibirsk: Publ. of SNIIGGiMS, 2003, 140 р.

2. Borodkin V.N., Kurchikov A.R., Kharakteristika geologicheskogo stroyeniya i neftegazonosnosti achimovskogo kompleksa Zapadnoy Sibiri (Characteristics of the geological structure and oil and gas potential of the Achimov complex of Western Siberia), Novosibirsk: Publ. of SB RAS, 2015, 300 p.

3. Nezhdanov A.A., Ponomarev V.A., Turenkov N.A., Gorbunov S.A., Geologiya i neftegazonosnost’ achimovskoy tolshchi Zapadnoy Sibiri (Geology and oil and gas potential of Achimov formation of Western Siberia), Moscow: Academy of Sciences Publishing, 2000, 246 p.

4. Nezhdanov A.A., Problemnyye voprosy stratigrafii mezozoya Zapadnoy Sibiri (Problematic issues of the stratigraphy of the Mesozoic in Western Siberia), Collected papers “Problemy stratigrafii mezozoya Zapadno-Sibirskoy plity” (Problems of the Mesozoic stratigraphy of the West Siberian Plate): edited by Gurari F.G., Mogucheva N.K., Novosibirsk: Publ. of SNIIGGiMS, 2003, 196 р.

5. Soin D.A., Skorobogatov V.A., Katageneric control over the formation and distribution of hydrocarbons deposits in the Achimovsky deposits of northern areas of the West Siberia (In Russ.), Vesti gazovoy nauki, 2014, no. 3(19), pp. 62–69.

6. Belonogov E.V., Pustovskikh A.A., Samolovov D.A., Sitnikov A.N. Methodology for determination of low-permeability reservoirs optimal development plan (In Russ.), SPE 182041-RU, 2016.

Authors

M. Bukatova, D. Peskova, M. Nenasheva, S. Pogrebnyuk, G. Timoshenko,
D. Solodov, V. Zhukov, A. Bochkov, Ph. D. in Geology and Mineralogy

Gazpromneft Science & Technology Centre (Gazpromneft STC LLC)

G. Volkov

Gazpromneft-Angara, LLC

A. Vashkevich   

Gazprom Neft, PJSC