Modern methods of hydrocarbon fuel prospects and exploration Methods of reservoir boundaries determination.
Abramov T.A., Skorodumov S.V., Lebedev E.S.
Tyumen State Oil and Gas University, Tyumen, Russia
genaevangel@mail.ru
The classic model of pressure behavior in well which works near the unsealing fault is solved by employing of special image well. This image well is located at the same distance from the other side of the boundary and works with same rate. The interaction of 2 wells could be described by applying of superposition. It is considered that applying of image well would give the same pressure behavior as we could receive in single well which flows near the boundary because of the same rates we’ve got no fluid flowing through the boundary.
The pressure build-up behavior could be also described by the applying of superposition. Equation of this superposition drawn in Horner plot gives the double-multiplied slope of build-up line of the late time in relation to the early time line. The distance to the boundary could be calculated by the equation:
,
(1)
where tx - the time of the intersection of the early and late times
Our calculations put in the main equation of the build-up shows an inexactness of the method of images. In our example we put the distance of 10 meters in the main equation of superposition of build-up and after that we drawn it in Horner plot. The time of intersection yielded the distance of 22,7 meters, which is larger more than in two times than initial. It could be described like that. In fig. 1 showed the depression surface of complex kind which yielded by the interaction of 2 wells – real and image (shown by the red line). But the functions describing processes after shut-in do not take into account the formation of complex surface and suit only to the single well. Consequently we offer a new more accurate calculation of the distance to the boundary based on new model of well producing next to the fault.
Fig. Formation of complex depression surface.
Seismic criteria for predicting reservoir quality and delineating complex traps in oxfordian sands of the West Siberian petroleum province
Kontorovich D.V.
A.A. Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia
clay@ngs.ru
Introduction
In the West Siberian petroleum province, common deep point (CDP) seismic reflection survey is the basic geophysical technique for prospecting and preparing oil and gas trap for deep drilling as well as for delineating the field outlines. The large amount of seismic exploration conducted during the past decade of the XXth century enabled the majority of medium- and high-amplitude traps to be revealed in West Siberia. Exploration for hydrocarbons showed that the major hydrocarbon pools found in Jurassic and Lower Cretaceous deposits have not only the structural but also lithological, tectonic and stratigraphic control. As predicted by geologists, the traps controlled by lithology and faults are likely to contain 30% of the forecast hydrocarbon resources.
Delineation of such exploration targets will require mapping the reservoir facies changes to impermeable rocks and conducting the quantitative permeability and porosity estimation.
In southeastern West Siberia the majority of hydrocarbon resources and reserves are associated with the Oxfordian reservoir, J1 horizon, Vasyugan Formation. Considered in the present study are the techniques that allow the reservoirs to be delineated and mapped using a combination of geophysical logging and deep drilling data. The field data are presented in the 2D CDP-time cross sections and in the well logs.
Lithologic characteristics
Callovian and Upper Jurassic sections of the study area are composed of the Vasyugan and Bazhenov Formations. The Vasyugan Fornation exhibits lacustrine-boggy, littoral and shallow depositional environtments. The Bazhenov rocks have been deposited in a deep sulfurous-contaminated basins enriched in planktonic organic matter. [1-3,5].
Lower Vasyugan Formation is subdivided into the lower and upper subformations. Lower Vasyugan subformation (Callovian) is made up mainly of mudstone with some sand and siltstone beds. The Lower Vasyugan subformation is 30-40 m thick, being 55-60 m thick in its lowermost parts. Upper Vasyugan subformation (Oxfordian) is a sequence of interbedding sandstone, mudstone and siltstone with coals and coaly mudstone. The full section of the Upper Vasyugan subformation has 4-5 sand beds, which form the J1 horizon productive in southeast West Siberia.
The presence of late regressive and early transgressive sandstone in the J1 horizon, allowed us to recognize two members (under-coal and over-coal) separated by a regional coal bed U1. A type section of the under-coal member is made up of J13 and J14 regressive sand beds separated by mudstone layer. The U1 coal bed corresponds to the maximum regression. This regionally persistent bed was deposited in a swamp coastal plain. The over-coal member occurs in the upper part of the J1 horizon between the U1 coal bed (inter-coal member) and Bazhenov base. The over-coal member contains transgressive J11 and J12 sand beds, which were deposited in deltaic and shallow epicontinental sea environments. The thickness of the member ranges from 5 to 35-40 m.
Bazhenov Formation (Tithonian) consists of black and brownish-black carbonate-siliceous-shaly organically rich rocks (up to 20%). The thickness of the formation, which is a regional seal for Upper Jurassic sandstones, is 10-30 m. The formation is the main chamber of oil generation in the sedimentary cover of West Siberian basin. For most West Siberia the Bazhenov formation is in the zone of peak oil generation [4,5].
Transgressive sand beds J11-2 of the over-coal member are exhibiting the most promise for petroleum potential in southeast West Siberian basin [5,6]. These beds are laterally irregular and often locally replaced by impermeable rocks that could have resulted in the formation of complex non-anticlinal traps. A similar situation is observable in a conjunction zone between Kaimysov arch and Yugansk mega-depression, where the majority of hydrocarbon accumulations are controlled by structural and lithological traps.
Trap mapping and successful exploration for oil and gas reserves in such geological setting will call for elaboration of the criteria for predicting zones of J11-2 sandstone spread, facies replacement and reservoir pinch-out as well as for evaluating their effective thickness with the use of the integrated interpretation of seismic, logs and deep drilling data.
Section acoustic properties, wavetrain IIa
Acoustic analysis of Upper Jurassic half of the section demonstrates that the Bazhenov rock have anomalously low P-wave velocities (2.7-3.0 km/sec). The Upper Vasyugan subformation has stable acoustic characteristics. The velocity gradient does not increase 0.2-0.3 km/sec in various rock types, except for the coal beds, in which P-wave velocities are lower (2.5-3.2 km/sec). In the Lower Vasyugan mudstones, the P-wave velocities have intermediate values compared to those measured for Bazhenov and Upper Vasyugan rocks. Velocity gradient at the boundary between Lower and Upper Vasyugan subformations is 0.2-0.5 km/sec.
The entire Upper Jurassic complex is reflected on time sections as a single composite wave (wavetrain IIa), mainly contributed by reflections generated on the Bazhenov top and base [7]. The impact of Vasyugan formation on the wave pattern lies in the complications of phases of composite reflection and changing its amplitude attributes. Silting or pinching out of the individual sand beds with the net thickness not exceeding 10 m leave unaffected the wave pattern that, in turn, impedes the forecasting of geological section of the Upper Vasyugan subformation based on seismic data.
Over-coal member net sand evaluation technique
As seen from wave pattern synthetics, the real influence on a shape of the interference signal and amplitude attributes of the wavetrain IIa might have been exerted by coal members, the reflected intensity of waves from which accounted for 25-70% of the wave energy generated on the Bazhenov mudstones.
Interpretation of geological and geophysical information showed that these tasks should be solved using indirect indicators and taking account of paleofacies depositional environments.
Bazhenov formation and U1 coal bed form the top and base of the over-coal member. These were formed in the periods of tectonic quiescence and were peneplanation planes. Thickness map for the over-coal member characterizes the U1 paleorelief in the time of Bazhenov deposition. Increased thickness of the over-coal member is typical of the Oxfordian paleolows, whereas decreased thickness is characteristic of paleohighs. The correlation chart (Fig.1) shows the wells aligned against the Bazhenov base and arranged with gradual decrease in the over-coal member thickness. The study shows that J11-2 sand beds with the enhanced net thickness were deposited within the paleoslope setting. This formation damages either towards the paleohigh crests or paleolow axes. Thus, predicting the over-coal member thickness will be enough to evaluate net thickness of J11-2 sand beds.
Other wave field parameters depending on a thickness of the over-coal member can be also chosen. The decrease in the over-coal member thickness can quantitatively evaluated from visual analysis of time sections. In addition, the thickness of the over-coal member can be determined using paleotectonic criteria.
Based on the seismofacies, paleotectonic information and amplitude attributes the modeling results allowed to generate an integrated parameter whose coefficient of correlation with over-coal thickness is 0.98. Standard deviation in thicknesses both designed and measured in wells does not exceed 1 m.
The map showing net sand thickness for the J11-2 bed has been generated based on the over-coal thickness map using amplitude attributes of wave field and the above relationship.
A comparison of the net thickness map with a structural map for the top of Oxfordian sands of the over-coal member enabled us to refine geological model of the discovered oil and gas fields and predict a series of the new structural-lithological exploration targets.
Conclusions
The whole body of geological and geophysical knowledge, an integrated approach to seismic, deep drilling and log data interpretation allows us to work successfully delicate problems of predicting geological sections within the Jurassic of West Siberia or develop geological models of a complex lithologically screened hydrocarbon accumulations.
The mapping techniques proposed for sandstone and permeable zones within the J11-2 beds, which are the most productive in terms of hydrocarbon reserves, have been very effective for a large number of prospects located in the central and southern parts of the West Siberian petroleum basin [7].
References
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