The gas-condensates-oil reservoir of the Karachaganak Field, Pri-Caspian Basin, Kazakhstan, consists of carbonates forming a low angle ramp in the Tournaisian-Early Visean, an isolated carbonate bank in the Late Visean-Bashkirian and a high-rising isolated build-up in the Early Permian (Asselian-Artinskian). The reservoir, sealed by Kungurian-Ufimian-Kazanian evaporites, is overlain by three salt diapirs that make the reservoir seismic imaging challenging and the use of pre stack depth migration seismic mandatory for interpretation.
Following the depth reprocessing of the seismic acquired in 1999, and as a consequence of the remarkably imaging improvements achieved, a revision of the Early Carboniferous reservoir stratigraphic architecture has been recently carried out. Its ultimate goal is to provide elements (internal stratigraphy, depositional facies and dolomite distributions) for the reservoir model updates, for the day by day drilling follow-up and for the future wells planning. The work integrated the definition of a 3D full field seismo-sequence stratigraphy framework with the analysis of the available wellbore data.
The Carboniferous interval, bounded by the Devonian-Carboniferous PV2 unconformity at the bottom and by the Carboniferous-Permian C1a unconformity at the top, was identified as a high rank sequence and internally subdivided in six mid rank sequences (Early Tournaisian, Early Visean, Late Visean, Serpukhovian I, Serpukhovian II and Bashkirian) bounded by mid rank sequence boundaries (PV1b - Top Early Tournaisian, Top Bobrikovsky, C7 - Top Late Visean, ILS – Intra Late Serpukhovian and C1b - Top Serpukhovian), either recognized from well data only or identified on seismic and correlated to unconformities recognised on wells.
Within this interval, the high rank transgressive section includes Tournaisian and Early Visean ramp carbonates delimited at the top by a regional marker of shale and cherty limestone that records the maximum sea level (C9 – Tula, high rank maximum flooding surface). The Late Visean and Serpukhovian high rank highstand section records the onset of biohermal deposits, and a phase of remarkable vertical growth, characterized by aggrading, prograding and downstepping internal geometries, well imaged by the reprocessed seismic depth cube. The Serpukhovian evolution has been mapped over the entire Field and the relevant seismic horizons (Top SERP AGG1, Top SERP AGG2, Top SERP PROG1, Top SERP PROG2, Top SERP PROG3 and Top SERP PROG4) have been interpreted, in terms of sequence stratigraphy, as the seismic response of low rank sequence boundaries marking the top of shallowing upwards cycles (recognized from logs and core facies interpretation). This was confirmed by the high resolution cross-well seismic profiles acquired along the northern margin of the carbonate bank.
The stratigraphic architecture presented in this study provides elements to explain the reservoir behaviour, and particularly the effects of dolomitisation.