Geology
The Post Salt Сomplex of the Precaspian Basin is the Future of Kazakhstan❜s Oil
Nikolay Matloshinskiy, Ph.D., Technical Director of Reservoir Evaluation Services LLP
Kairat Adilbekov, Advisor to the Director for Geology of BSG OIL LLP
Roman Matloshinskiy, Ph.D., chief geoinformation system specialist of Reservoir Evaluation Services LLP
Natalia Suyarkova, Chief Geophysicist of Reservoir Evaluation Services LLP
A new stage in the study of the salt-dome part of the Precaspian Basin is associated with the widespread use of 3D seismic exploration. The prospectivity of deep horizons of the postsalt complex are determined by the capacity of the traps; being on the paths of hydrocarbon migration from the sub-salt source to the upper horizons, the traps inevitably had to be filled with migrating hydrocarbons.
Introduction
The oil and gas industry of the Republic of Kazakhstan is currently going through another difficult period, when the reserves of the currently developed giant fields will last for the next 15-20 years, but it is not clear which areas of exploration should be associated with providing the industry with hydrocarbon resources in the future. Of these, this article examines the post salt complex of sediments of the Precaspian basin, thanks to which, 125 years ago, Kazakhstan became one of the oil-producing countries. Few people consider the post salt complex as a possible source of hydrocarbons that will ensure the growing needs of the Republic for many years to come. Meanwhile, the recognition of the fact of the subsalt genesis of oil and gas and the presence of their deposits in the upper horizons clearly indicates that at the same time all traps, if they exist on the way of hydrocarbons migration from subsalt deposits to the upper parts of the post salt complex, must be filled with hydrocarbons in accordance with Gussow’s law.
There is a certain contradiction in the oil and gas potential of the post salt complex that lies in the fact that in the inner part of the depression, where the hydrocarbons generation and the activity of salt tectonics should be most pronounced, oil deposits in the upper part of the post salt complex were practically not revealed. Years of exploration here despite the numerous signs of oil in the core of drilled wells [4] resulted only in discovery of several small fields. The reasons for the absence of oil discoveries here may be related to the fact that oil could accumulate in deeper horizons, and it did not reach the traditional traps in the Cretaceous, Jurassic and Upper Triassic, and the signs reflect only the "breathing" of these undiscovered deposits. However, this requires the presence of capacious traps in the lower and middle parts of the post salt complex that still needs to be confirmed.
The level of study of the issue
Fundamental works of the following scientists form the basis of modern ideas about the structure of the salt dome part of the Precaspian basin: N.V. Nevolin (1951) with co-authors, N.A. Kalinin (1958), P.Ya. Avrov with co-authors (1935, 1959) and Y.A. Kosygin (1960) where the data accumulated by the second half of the last century have been summarized. Kazakhstani researchers have made a significant contribution to the study of the salt dome tectonics of the Precaspian Basin in the modern era, namely E.K. Aznabayev, E.S. Votzalevsky, Y.A. Volozh, B.A. Yeskozha, K.H. Bakirov, G.ZH. Zholtayev, A.V. Matusevich, T.A. Oshakpayev and many others. It should be especially noted the great importance in the study of salt dome tectonics of the III All-Union Symposium held in Almaty in 1969 [7]. The symposium finalized the peculiar results of the long-term study of salt dome areas, in particular the Precaspian basin, since it’s holding coincided with the beginning of the discovery of deposits in subsalt deposits; that is why the relevance of the post salt complex here has significantly decreased for many years. The existing ideas about the salt dome part of the depression are based on seismic and other data from the very initial stages of their application, when the work (reflection seismic survey, CDPM, drilling) was mainly focused on the apex parts of the domes.
At the beginning of this century, a group of specialists of FIOC company (B.A. Yeskozha, M.S. Trokhimenko, G.V. Voronov, and others) under the leadership of the company's vice president for exploration, Ray Leonard, established the patterns of hydrocarbon distribution, in the so-called ‘Middle Triassic’ or structural stratigraphic (structural sedimentation) traps [3, 8]. However, it was quickly established that not all such traps can contain oil and gas deposits, and those that contain deposits, as a rule, are characterized by complex reservoir development that led to a reduction in reserves. This direction, however, turned out to be very productive and only FIOC alone identified more than a dozen fields, but, due to their small reserves, it was not possible to significantly change the situation with the increase in booked reserves. At the same time, this trend attracted the attention of investors and in recent years, post salt plots have been actively sold at auctions held in the Republic.
The most complete current state of the issue is presented in the work of M.P. Antipov and Y.A. Volozh [2], that examines the structural features and oil and gas potential of the post salt section of the Precaspian basin, and which somehow sums up the traditional approach to this problem. The authors' fair statement that "the productivity of salt structures is selective and there are no obvious patterns in their distribution" (p. 132) shows that the traditional approach has practically exhausted itself here. The proposed by the authors maps of three prospective complexes without a local forecast do not cause much enthusiasm.
Factual data and study methods
It can be stated that the geological prospecting and exploration practice has come close to the need for further disclosure of the hydrocarbon potential of the post salt complex of the basin in new conditions, at a new level (3D seismic). Considering the problem, among other things, in the light of the doctrine of hydrocarbon system, that is, considering all the components of the process: from hydrocarbons generation, migration and accumulation, including also preservation, one can count on drawing up a complete picture of the prospectivity [5, 6]. The results of 3D seismic surveys obtained not so long ago on large areas (Tassym, Zharkamys, Taisogan and others) have not yet become the subject of wide discussion, and in many ways, they are only being studied. At the same time, the importance of these 3D surveys for studying salt domes in their entirety, especially the lower parts of the post salt complex, the internal structure of salt cores, salt-postsalt transition areas with all the details of the structure, can hardly be overestimated.
Use of this data should lead to breakthroughs in the general understanding of the problem. The key aspect here is the prospectivity for oil and gas potential, determined by the fact that in salt-free troughs, subsalt hydrocarbons themselves tend to get to the depths available for modern drilling, migrating through windows and filling various traps on their way [5,6], primarily related to the closed isolated nature of the troughs at a depth of 3.5–5.5 km. During migration, hydrocarbons are always purified from aggressive components (hydrogen sulfide, etc.), due to binding by iron oxides, which are widespread in Permian-Triassic sediments [1].
The study method consists in the correlation of seismic horizons and, thus, the study of the structural features of salt domes and inter-dome troughs using 3D seismic data, for the purpose of predicting the development of reservoirs, traps and salt-free windows in them. Figures 1 and 2 show examples of such studies and correlations of horizons. The main task was, based on accumulated data, by the nature of seismic recording to learn how to recognize the position of salt-free windows and to identify various sections of Triassic and Upper Permian deposits. The figures show two troughs with Permian and Triassic depocenters, for both of which the presence of salt-free windows is established. Salt-free windows are easily recognized by the characteristic reduction in the thickness of subsalt deposits due to their dehydration in contact with post salt reservoirs rocks [5].
The results obtained
Despite the presence of the salt-free windows and the obvious direction of hydrocarbons migration due to the unilateral rise of sediments, the task of predicting possible hydrocarbon deposits does not always look easy here. In the first case (Figure1), it is obvious that accumulations can be expected in overhang conditions in the Upper Tatarian reservoirs, in the second (Figure 2) no obvious reflections can be traced at the boundary of the trough and salt, and in Triassic sediments on contact with salt no signs of possible accumulations are visible. Meanwhile, obviously in the salt body reflections resembling the structure of the trough can be traced. At the same time, the intra-salt throughs (on the right and on the left) are in no way related to the inter-salt trough. As can be seen in Fig. 2, their edges rise towards each other. The troughs buried in the salt mass can be considered as detached parts of a large trough. That can be seen from the similarity of reflections of Triassic sediments and intra-salt parts of a large trough (Figure 2).
Currently, they are separated from each other due to the peculiarities of the development of the salt domes tectonics that consists in the active pressing out of salt under the intra-salt parts of the trough that is why these parts of the trough have significantly sunk down, breaking off from the main trough. At the same time, the salt penetrated into the separation zone and formed a narrow salt wall that at the same time is the slope of the dome and separates the buried parts of the trough. The question of whether the hydrocarbons could get into the inner-left part of the trough remains open, even in the presence of the salt-free window. However, assuming that oil generation began in the Early Triassic and the salt-free window already existed by the end of the Cretaceous, overflows could occur, filling Triassic sediments as good reservoirs. The reformation with separation and deep immersion of a part of the trough and the formation of a salt wall could take place in the Triassic (on the right in Fig.2) or associated with the main periods of tectogenesis - Mesozoic at the end of the Cretaceous and Alpine - in the Miocene (on the left).
Obviously, overhangs are a more interesting area of exploration. The problem with overhangs lies in the fact that quite active searches for hydrocarbons in sub- overhang traps in the 80-90 years of the last century did not lead to significant success, and as a result, many dry
Fig.1. A fragment of a time section demonstrating the structure of the Permian-Triassic trough. The inset shows the position of the study area (a red square) on the map of salt domes according to A.V. Matussevich and others
Fig.2. A fragment of a time section demonstrating the structure of the Triassic trough. Red arrows indicate the movement of salt; green arrows indicate the position of salt-free windows. The red rectangles show a fragment of the intra-salt trough that is compared with the inter-salt one (Т2)
wells have been drilled. From today’s perspective, it is clear that the exploration was conducted based on the results of Kirchhoff's time migration that objectively images the structure if there is no significant lateral velocity variability (Kobet, 2010). If there is such a variability that is obvious for clastic deposits in salt, the sub- overhang part may occupy a completely different place than that, which is visible on time sections. Here, prestack depth migration is required with the construction of depth/velocity model in order to accurately determine the position of the overhang that can shift up to 3-5 km from its position on the time section.
Overhangs (in English-language literature, "overhangs" actually stand for both overhangs and wings [10]) are the most pronounced and indicative cases of the presence of post salt sediment packages inside salt bodies. Their formation is associated with salt in the vaulted parts of the domes getting to the surface and the formation of salt glaciers or salt lakes with redeposition of salt on the surface from the dome tops to the lower areas [9, 10]. With the beginning of a new phase of sedimentation (sea level rise), such salt accumulations have been overlapped by clastic sediments, forming well-expressed overhangs. Salt lakes are quite widely present in the depression at the present time (Elton, Baskunchak, Inder, Chelkar, etc.) where salt redeposition is currently happening or may be happening. When it is overlaid by clastic rocks, new overhangs will be formed over time as a result of a new transgression. In Permian-Triassic, this process has been many times more widespread and repeatedly repeated.
According to seismic data, impurities of non-salt material are noted in almost all salt domes in the form of reflections of different lengths or their packages, which are not always separated from the trough by salt walls. Intra - salt inclusions are most pronounced at the base of salt domes and near their edges. The formation of overhangs within the domes can be demonstrated by the example of an inline fragment, where the slope of the salt dome is presented and it is clearly visible that all alien inclusions in salts correlate well with the main stratigraphic boundaries of the trough, reflecting the main interruptions in sedimentation (Fig. 3). It was exactly to the interruptions in sedimentation that salt exits to the surface and the formation of a overhangs in the form of salt redeposition over a large area were timed, as was the overlap of salt deposits coming to the surface with clastic material at the beginning of the next sedimentation cycle. In other words, such a picture may mean the pulse-cyclic activity of salt that begins to move at the beginning of the next sedimentation cycle (a new rise in sea level).
The consistency of the reservoirs should hardly be questioned, all more or less thick sections falling into the sub-overhang conditions are no different in any way from similar deposits in the trough, since they were formed under absolutely the same conditions. Moreover, intensive subsidence, and therefore sedimentation, will actively occur near the dome, where there is enough salt for its outflow. Given the formation of sections due to fluvial transfer of material in the era of its greatest influx, it will deposit massively where there is active subsidence that means near salt domes. Overhang salt behaves like ordinary deposits and is not involved in halokinesis.
Fig.3 A fragment of a time section showing the structure of one of the slopes of the salt dome. The sub-overhangs are timed to coincide with the main breaks in sedimentation
Conclusion
The Precaspian basin is one of the epicenters of the concentration of hydrocarbon resources of the Globe, due to inexhaustible generation capabilities, due to the widespread development of organically rich deep-sea depression Paleozoic strata covering most of the depression, as a fragment of the Late Paleozoic Ocean. Simple calculations, considering the impossibility of long-range migration of hydrocarbons through depression deposits into reservoirs of buildups (Tengiz, Kashagan, Karachaganak, etc.), show that the undiscovered potential exceeds known reserves by at least an order of magnitude [5]. This potential would have remained in the form of shale oil and gas at great depths in the sediments of the Late Paleozoic depression, if not for the salt dome tectonics with its salt-free windows that ensure the hydrocarbons migration into Permian-Triassic and younger sediments. Thus, favorable conditions have been naturally created in the depression for the widespread migration of hydrocarbons from subsalt generating strata, through numerous windows into the post salt strata of inter-dome basins with the hydrocarbon’s purification from aggressive components, primarily hydrogen sulfide, due to migration through the red-colored strata of the lower part of Permian-Triassic section.
In this schematic form, this idea looks very attractive and promising, but the main problems are that to date, not only the scale and phasing of hydrocarbons generation have been poorly studied, but nothing is known about migration pathways, and the features of reservoir development in Permian-Triassic sediments of the troughs remain poorly unexplored. Yes, assumptions on this matter and some calculations exist, but this knowledge creates the same illusion of knowledge that accompanies the development of the hydrocarbon resources of the post salt complex throughout its history. Thus, the widespread idea of poor reservoirs in Permian-Triassic sediments is also based on the study of the initial stage, when Upper Permian molasses deposits have been studied in the eastern part of the depression. Now it has been established that already within the limits of the Karatobe Yuzhniy deposit, Kazanian sandstones have good reservoir properties. With their further transfer westwards, they will become more and more washed from the clay component and sorted sandstones with good reservoir properties. The same applies to the Triassic deposits.
The conducted study of the 3D CDPM seismic survey data showed that there are grounds for some optimism regarding the development of Permian-Triassic reservoirs in the middle and lower parts of the troughs. First of all, this is evidenced by the development of Triassic and younger depocenters, which are troughs with active deflection (salt outflow) in the Triassic time and later. The second factor is the more complex history of the development of salt domes with the separation of the marginal part of the trough and its subsequent immersion with the formation of salt walls in the separation zone. And finally, the prospectivity of salt overhangs overlapping reservoirs and their association with the interruptions in sedimentation, all this, as well as many other aspects, for example, the better washing out of Permian-Triassic sediments in the western direction, allow us to count on the presence of reservoirs in the middle and lower parts of the troughs.
As for the new data on the features of the structure of salt domes, only a tiny part of the shaft of information that has already been received will only expand due to large-scale 3D seismic surveys on different domes and their groups is provided. At the regional level, such data needs detailed study and comprehensive discussion in order to develop an updated reliable understanding of the structure, development and oil and gas potential of the salt dome part, especially its deep horizons. The correct definition of the directions of the study process, a clear formulation of the primary urgent tasks along the way, ensuring access to the data for all interested in solving the problem — these are the main conditions for the successful development of huge resources that may hide in the lower horizons of the postsalt strata.
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