Lithofacial interpretation of well-logging data in the interval of terrigenous deposits of the J2 horizon

Levshakov A.M., Glinskikh V.N., Pavlova M.A.

A.A. Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia

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Nowadays great attention is paid to exploration and study of deposits with hard-to-recover reserves in the Mid-Ob region. One of such objects is the producing horizon J2 which is characterized by complex geology and low values of porosity and permeability. Lithological inconsistency of aleurolitic rocks and their replacement with argillaceous deposits causes big difficulties when defining reservoir rocks [1, 2]. Predicting areas of improved reservoir characteristics is based on lithological and petrophysical core analysis and following reconstruction of sedimentation conditions. This work aims to develop approaches to the problem of lithofacial interpretation of well-logging data, which provide the basis for paleoreconstructions of the depositional environments in case of absence of core data.

Rock types of terrigenous sediments of the J2 horizon in the East Surgut field, addressed to in this work, are represented by variously grained sandstones, aleurolites, argillites and coals. Sedimentation environments established from the core data, are divided into three groups: continental, transitional and marine. They are mentioned in the stratigraphic order, specified by transgression, which took place in the study area during the time of formation of J2. The continental group is represented by alluvial complex environments: meanders, channels, shallow river beds, floodplains and swampy floodplains, fractured debris cones and also by lacustrine complex environments: coastal and central parts of the lakes. Transitional group includes environments of coastal, natural and washed-in levees, lagoons, inter-ridge ravine, marshes and coastal plains. Marine facies are represented by beach and shelf. Facies of channels and meanders have the best reservoir properties [1-3].

Fig. Comparison of the results of lithofacial interpretation of the core data and the well-logging data.

Lithofacial analysis of well-logging data is based on detection of relations between geophysical characteristics and sedimentation environments reconstructed using core analysis. The initial materials are well-logging data, and those of petrophysical, lithological and facial analysis of the core obtained from the wells of East Surgut field. The main stages of the data pretreatment include estimation of well logs quality, their correlation and depth adjustment with the core, and transformation of well-logging methods signals into neutron porosity (W), alpha parameter of spontaneous polarization (αSP) and double-differencing parameter of gamma ray logging (JGR). With the use of reference wells data, the distributions of geophysical characteristics αSP(JGR), JGR(W), JGR(ASL), JGR(GGDL) were built for the intervals of the established lithofacies. On the basis of determination and analysis of these relations boundary conditions for every lithofacies were found and lithofacies were quantitatively described. The algorithm for lithofacial analysis of well-logging data was developed and programmed. SP and GR methods have proven to be the most important because they allow to reliably distinguish continental alluvial lithofacies, which have the best filtration-capacitive properties. Marine lithofacies located in the uppermost part of geological formation and containing marking layers of glauconite streaks are effectively defined by lateral logging data. The most difficult part appears to be the separation of transitional lithofacies group and lithofacies of lacustrine complex due to similar values of their geophysical characteristics. In order to achieve higher reliability, wide range of methods was used: spontaneous polarization, gamma ray, acoustic, neutron-neutron and gamma-gamma logging. Ambiguity of interpretation results leads to each interval being associated with several lithofacies. Additional application of statistic approaches allows to characterize lithofacies providing probabilistic estimates.

The algorithm has been used for lithofacial interpretation of well-logging data from boreholes in the East Surgut field. The results of building the images based on the reference wells correlate perfectly well with the lithological core analysis. The developed algorithm allows to conduct an operative lithofacial analysis and enhances reliability of paleoreconstructions of depositional environment.

References:

1.Kontorovich A.E., Vakulenko L.G.,Kazanenkov V.A. and others. Sedimentogenesis of reservoirs of mid-upper Bathoniannd their petroleum potential in the Mid-Ob (2010).Geology and geophysics, volume 51, №2, pp 187-200.

2.Kontorovich A.E., Nesterov I.I.,Salmanov F.K. and others (1975). Oil and gas geology of West Siberia. Nedra. 679 pp.

3.Barkov S.L. Depositional environment and geological and commercial models of heterogeneous reservoirs of oil and gas deposits of the Mid-Ob (1999). Edited by Kanalin V.G. 122 pp.

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