EnForce Energy Corp.
New York
A line pattern of well cluster spacing was chosen in western Siberia because of taiga, marshes, etc., on the surface.
The zoning of the oil pools within productive Upper Jurassic J3 intervals is complicated. This is why until the early 1990s almost each third well drilled in the Shaimsky region on the western edge of the West Siberian basin came up dry.
The results of development drilling would be much better if one used some sedimentological relationships of zoning of the reservoir rocks within the oil fields. These natural phenomena are:
- Paleobasin bathymetry
- Distances from the sources of the clastic material, and
- Proximity of the area of deposition.
Using the diagrams in this article, one can avoid drilling toward areas where the sandstones pinch out, areas of argillization of sand-stones, or where the probability of their absence is high.
BASIN EVOLUTION,
LOCAL SEDIMENTOLOGY
The late Triassic T2 and early Jurassic J1 sequences at the always elevated Shaimsky region of the West Siberian basin contain mostly effusive deposits. Those can be seen on seismic sections (Fig. 1A, left half below 1.6 sec).
During the Middle Jurassic J2 age, the continental sedimentary systems were formed. The Tymen formation (J1-2 tm) is oil saturated all around the West Siberian basin. Because of the hilly topography of the Shaimsky region at those times, the deposits of the Tymen formation aren't developed anywhere across the described region.
The Upper Jurassic J3 clastic deposits are oil saturated all around the West Siberian basin including the Shaimsky region. Here the oil pools are confined to sand-siltstone beds deposited by wave processes around the islands of a large paleoarchipelago.
These sand-siltstone beds are the lowest sequences of the late Jurassic age (J3 dn1), and then, belong to the coastal sedimentary systems. Later, during a transgression, they were overlain by shales of the Verkhnedanilovskaya (J2 dn2) subformation, which should be classified as a shallow shelf sedimentary system.
While the regional transgression continued, the deep sea sedimentary system (Bazjenovskaya formation J3b) was formed all over the West Siberian basin.
The North Danilovskaya brachyanticline, which is a part of a large Shaimsky arch, was an island during the late Jurassic J3 transgression and had rugged topography. It is confirmed by a lateral onlap in the area of wells 6 and 8, for example (Fig. 1A).
The same conclusion comes after a study of other regional and local seismic lines across all this region. The locations of seismic lines across the North Danilovskaya oil field are shown in Fig. 1B, and this is the real basis for mapping those islands.
The erosion products from the islands were transported to a shallow sea and sorted under the influence of the following factors:
- the Paleobasin bathymetry. The deeper the area was, the more shale material the deposits contained.
- the distance from the source areas (islands). The closer to the island the area was, the more coarse the deposits were.
- the proximity of the sedimentary area. In a closed area, only pelitic fractions could be washed out and carried away.
A sketch map obtained from both seismic and well logging data and showing the location of islands of this part of the Shaimsky paleoarchipelago is given in Fig. 1B.
A geological cross section showing the formation of sand reservoirs obtained from well data is given in Fig. 1C; the zero line corresponds to the level of the paleosea. According to this sketch map and cross section, sand reservoirs pinch out toward the paleoislands and pass into shale rocks downdip.
RESERVOIR ROCKS
Core analysis and well log data from more than 100 wells across this region show that four different classes of reservoir rocks produce oil. Their characteristics are as follows:
/First class reservoirs contain coarse grained sandstones and pebbles and less than 25% shale material. An average porosity of these rocks is more than 20%, and permeability is high enough.
/Second class reservoirs contain well grained sandstones and less than 20% shale material. An average porosity of these rocks is up to 20%, and permeability is high enough.
/Third class reservoirs contain sandstones and siltstones and up to 35% of shale material. Porosity of these rocks is less than 13%, and permeability is low enough.
/Fourth class reservoirs contain siltstones and up to 65% of shale material. An average porosity of these rocks is less than 9%, and permeability is low.
The gross pay thickness varies from zero to 28 m within the lower section (J3 dn1) of a clay sediment mass (Danilovskaya formation J3 dn), as was mentioned above. The thickness of this receptacle itself varies from 21 m to 126 m.
The reservoir rocks are missing where Danilovskaya formation thickness is more than 114 m thick or less than 40 m (Fig. 2A). This is why it may be easily supposed that during deposition of the Lower Danilovskaya formation (J3 dn1) the depth of the paleosea was something like 40 m less than the thickness of the whole Danilovskaya formation.
RESERVOIR ROCK ZONING
Generally the deeper DELTA H the sea around the paleoislands, the more the gross pay thickness (h) is (Fig.
At the shallow parts of the paleosea, the gross pay thickness is less than 10 m, but mainly the reservoir rocks of first and second classes form the pay sections here.
Those area in which the thickness of the Danilovskaya formation is between 40 m and 60 m (the paleosea depth was less than 20 m) are surrounding the former islands. The wells hitting the tops of these former islands come up dry because they miss reservoir rocks. One example is well 7 (Fig. 1A).
These wells that meet the reservoir rocks at the slopes of the former islands will have oil influx rates of about 50 metric tons/day, such as well 4 in Fig. 1C, even though the gross pay thickness is less than 10 m there.
These zones (islands and their slopes) inside the North Danilovskaya field are shown at the sketch map (Fig. 1B).
Figs. 1C and 2A also show that at the areas where the thickness of the Danilovskaya formation becomes about 110 m (the paleosea depth was less than 70 m), the lithologic replacement occurs and this formation doesn't contain reservoir rocks any more. The contours of this lithological replacement, which are the oil field boundaries, are shown in Fig. 1B.
Both for development drilling and injection projects it is necessary to predict the properties of the reservoir rocks and their spacing. The diagrams in Fig. 2B, C, and D help to establish the distribution of the reservoir rocks of different classes across the oil field in connection with the natural phenomena mentioned above.
While such phenomena as distance from the source of clastic material and paleosea depth don't need any additional explanations, the other phenomenon (proximity of the sedimentary space) does. An open horizon includes 360 from some point of a surface. Within an archipelago, the sea surface is partly screened by some islands.
This portion can be counted and expressed in degrees for any point of the sea surface. These figures were used to construct the diagrams in Figs. 2B and D.
The diagram in Fig. 2B not only confirms that the more closed the deposition space was and the closer to the source it was located, the better are the properties of the reservoir rocks, but also gives a concrete realm of distribution of reservoirs of the first and second classes.
The best reservoirs are located in places that were closed for as much as 120-310 and weren't farther than 1.3 km from the islands. There is also a limitation of the paleosea depth (about 50 m) for the distribution of reservoir rocks of the first and second classes (Figs. 2C and D).
The data concerning the North Danilovskaya field only were used for the construction of the diagrams in Figs. 2B, C, and D. The data from the other oil fields (Danilovskaya, Tolum, etc.) located in the described region were added for the construction of the diagram at Fig. 2.A.
All these diagrams are in compliance with each other. This means that the diagrams in Fig. 2 are adapted to the conditions of the whole Shaimsky region (western edge of the West Siberian basin).
CONCLUSIONS
Using the diagrams in Fig. 2, one can predict the gross pay thickness and quality of the reservoir rocks in any place of the oil field. The prognosis of gross pay thickness can be done along the seismic lines, where the distance between the marker "B" and the sole of the pay zone may be counted.
At the areas between seismic lines, the map of this distance should be used because this map is more reliable than the isopach map.
The contours of the paleoislands obtained from seismic and also well log data should be mapped in order to predict the quality of the reservoir rocks. Then the distance from a planned well's sedimentary area should be measured. The expected class of reservoir rocks will be established simply by comparing these figures with the diagrams.
About 40 wells were spaced within North Danilovskaya field using these diagrams, and only two of them were dry holes.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
