Geology
A New Vision of Oil and Gas Prospectivity in the Northern Caspian Shelf
Matloshinsky N.G., Golmstock А.А., Matloshinsky R.N., Aytkalieva N.B., Syarkova N.A., Kolomatskaya O.V., (RES LLP), Adler V.M. (Veritas Caspian LLP)
Introduction
Geological exploration in the Kazakhstan Sector of the Caspian Sea (KSCS) has been actively pursued since the Republic of Kazakhstan gained independence. The initial foundation for these efforts was established through extensive 2D seismic surveys conducted across the entire shelf by the Kazakhstancaspishelf (KCS) consortium, with related interpretation results released from 2000 onwards [3]. Over the past three decades, exploration has yielded both remarkable successes (the most impressive of which was the discovery of giant and medium-sized reef-type fields such as Kashagan, Aktoty, Kairan and Southwest Kashagan) and a wealth of geological and operational experience that requires re-evaluation to determine future development strategies.
From today’s perspective, it is evident that no additional reef-type prospects comparable to the above discoveries have been delineated within the northern part of the shelf. Likewise, many of the potential traps within the postsalt section of the northern shelf that were originally identified in 2000 and repeated in the literature [8] have proven to be of limited economic interest. Over time, exploration focus shifted toward the Mesozoic (Jurassic) section of the collision zone, where relatively shallow accumulations (1,500-2,500 m) have been discovered, including Kalamkas More, Auezov, Khazar, and Zhambyl. However, exploration drilling at other structures, such as Satpayev, Abay, Zhetysu, and Kurmangazy, failed to yield commercial hydrocarbon flows.
In the central and southern KSCS, numerous structural closures, including large-scale features, have been mapped. Despite extensive detailed 2D and 3D seismic coverage and the drilling of several wells (Atash-1, Tyub-Karagan-1,2, Al-Farabi-1, Yu-Nursultan-1, Zhenis-1), no commercial discoveries have been made [6]. These results underscore the need for a comprehensive reassessment of the region’s hydrocarbon potential and the formulation of a revised exploration concept reflecting the current level of geological understanding.
A significant contribution to recent exploration efforts has come from Veritas Caspian LLP, part of Viridien (formerly CGG), a global leader in earth sciences, exploration geophysics, subsurface imaging, and digital transformation of geological datasets. Between 2006 and 2009, Veritas Caspian acquired over 8,000-line km of speculative 2D seismic data across the Caspian shelf.
Abstract: Geological exploration in the Kazakhstan Sector of the Caspian Sea (KSCS) has been ongoing for more than three decades, leading to the discovery of several significant fields in the northern region – notably Kashagan, as well as the drilling of numerous dry wells. Consequently, investor interest in KSCS has declined in recent years. Renewed exploration success now depends on a critical reassessment of existing results and a redefined understanding of the region’s hydrocarbon potential. This study presents one such reassessment, integrating geological and geophysical data with newly reprocessed seismic data from the northern KSCS. The reprocessing utilized prestack depth migration (PSDM), enabling improved subsurface imaging and interpretation. The results reveal enhanced prospectivity in the presalt Paleozoic section, including the identification of approximately two dozen likely organogenic-detrital type structures with estimated recoverable resources of 1,550 MMT. Additionally, a series of Triassic structures are located within saltfree troughs (Triassic depocenters) at technically viable depths of 3-4 km or greater and may collectively contain up to 365 MMT of recoverable oil. Of potentially greater strategic significance is the interpreted southern margin of the paleo-Caspian basin, where reef complexes developed during Carboniferous and Permian periods occur within the present-day back-thrust zone. In this structurally complex area, characterized by Paleozoic carbonates divided into blocks, seven large structures have been delineated. Preliminary estimates indicate that the total recoverable resources in the underthrust Paleozoic carbonates zone may be several times greater than the reserves of Kashagan.
The seismic data originally processed by KCS in the late 20th and early 21st centuries using pre-stack time migration (PSTM) methods are now considered technically outdated. For the northern Caspian, where complex salt tectonics significantly distort subsurface imaging, pre-stack depth migration (PSDM) is essential for accurate delineation of the presalt section. Historically, however, PSDM was rarely applied to regional 2D datasets. Under an agreement with the Government of the Republic of Kazakhstan, Veritas Caspian LLP and Reservoir Evaluation Services LLP (both part of Viridien) undertook a large-scale reprocessing project, applying PSDM to a substantial portion of the original KCS seismic database across the entire KSCS.
Recognizing that the commercial value of seismic datasets depends on a clear understanding of the prospectivity of the areas they cover, Veritas Caspian engaged Reservoir Evaluation Services LLP, an organization with extensive experience in seismic and well data interpretation across all of Kazakhstan’s hydrocarbon basins to interpret the newly processed depth data for the northern KSCS. This ongoing project commenced in 2024, and the present paper provides a preliminary summary of its early findings.
Scope of study and methodology
The interpretation presented in this study is based on approximately 5,000-line km of 2D depth-migrated seismic data acquired within the northern part of the Caspian Sea. These lines were originally processed by Kazakhstancaspishelf (KCS) in the late 1990s and subsequently reprocessed by Reservoir Evaluation Services LLP (RES) in 2023-2024 using the pre-stack depth migration (PSDM) algorithm (Fig. 1). Data from 11 exploration wells were integrated into the seismic interpretation to ensure accurate seismic-to-well ties.
Fig.1 Basemap of PSDM processed 2D seismic lines (red color) for the northern part of the Caspian shelf. The thick purple line is the position of the northern boundary of the collision zone. The inset shows a map of shelf oil fields according to [2].
Application of PSDM improved seismic resolution and enhanced the image of lower Paleozoic reflectors. This, in turn, allowed for a substantial refinement of the existing structural model, revealing previously unresolved features and enabling more detailed geological interpretation.
Stratigraphic calibration between well data and seismic profiles was achieved through the generation of synthetic seismograms derived from sonic and density logs. A zero-phase wavelet extracted from the reprocessed dataset was applied in synthetic generation to ensure optimal tie quality.
Seismic interpretation involved correlation of 11 key stratigraphic horizons, including three within the Mesozoic section (III, V, T2) and eight within the Paleozoic section (VI, VI-1, P1, P2, P21, C1, P3, PZ). Structural maps were generated for all presalt Paleozoic horizons using Petrel software. An example of horizon correlation and fault delineation is shown in Fig. 2.
Fig. 2. Fragment of a seismic section in depth with horizon correlation. The position of the seismic line is shown by the red line in Fig. 3.
In addition to structural mapping, dynamic interpretation was conducted to characterize reservoir potential. This included targeted analysis of seismic attributes within key intervals of interest, primarily to refine delineated prospects alongside both conventional and clusterbased seismic facies analysis across the entire study area.