How Seismic Can Contribute to Sequence Stratigraphy of Deep Buried Carbonate Banks. The Example of the Late Paleozoic Karachaganak Field (Kazakhstan)

Luoni, Fabio ¹; Borromeo, Ornella ¹
(1)eni E&P, San Donato Milanese, Italy.

The gas-condensates-oil Karachaganak Field is located in the northern part of the Pri-Caspian basin, NW Kazakhstan and consists of Carboniferous-Early Permian carbonates.

A good example of using a depth seismic dataset to describe the carbonate bank internal geometry is the 3D full field seismo-sequence stratigraphy interpretation of the Early Carboniferous reservoir.

The seismo-sequence stratigraphy interpretation comprised, first the recognition of seismic sequence boundaries (SB) and other stratigraphic surfaces (maximum flooding and tansgressive surfaces). This phase was carried out by combining seismic PSDM volume interpretation in a 3D environment with line drawings along selected profiles. Then, the identified surfaces have been mapped in detail over the Field and the resultant geometrical framework provided valuable indications on the evolution of Karachaganak bank. The Field-scale consistency between seismic SB’s and unconformities identified on cores or interpreted on logs was achieved by a systematic well to seismic tie. Seismic forward modeling and cross well seismic profiles were also used during the interpretation.

Within the Carboniferous seismic sequence (bounded by the Devonian-Carboniferous and Carboniferous-Permian SB), the Late Visean and Serpukhovian record the onset of biohermal deposits and a phase of remarkable vertical growth. In the Serpukhovian, seismic scale mound complexes have been recognized and the aggrading, prograding and down-stepping phases have been mapped over the Field. Within this geometrical framework, the facies depositional model sees predominantly microbial bioherm deposits (in situ and reworked) and subordinately bank interior skeletal grainstone and packstone, that occur in the uppermost part of the sequence in the centre of the Field.

In the Serpukhovian section, the vertical growth of seismic scale mound complexes (aggradation) shaped a substantial relief which was maintained, though reduced, during the subsequent lateral expansion (progradation) and up to the time of a seismic SB, as documented by onlaps identified on seismic and correlated with an unconformity recognized on core.

The geometrical framework inferred from seismic makes Karachaganak an example of a Paleozoic carbonate bank dominated by microbial bioherms, significantly different from the classical flat-topped platform model.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.