Using Chemo and Magnetostratigraphy to Define a Chronostratigraphic Framework in an Isolated Carbonate Platform: The Tengiz Field, Republic of Kazakhstan

Ratcliffe, Ken T.; Urbat, Michael; Davies, Emma; Playton, Ted; Katz, David A.

Defining chronostratigraphic correlation frameworks in carbonate reefal platforms is problematic. Typically, biostratigraphy is hampered by strong facies control and recrystallization, and palynomorphs are frequently oxidised. When in the subsurface, log signatures can be misleading and tracing seismic reflectors challenging. Additionally, when dealing with cuttings samples, recognition of facies stacking patterns and prediction of stratal architecture is impossible.

Tengiz Field is an isolated carbonate platform that developed in the southern Pricaspian Basin, Kazakhstan, from Late Devonian through Carboniferous times. The temporal and spatial variability inherent to isolated platforms and the differential development of seismically-transparent microbial boundstone-dominated upper slopes during various stages of growth at Tengiz add to the stratigraphic correlation difficulties outlined above. However, understanding the age relationships and stratal architecture of the boundstone-rich slopes at Tengiz are critical for hydrocarbon production.

Here, results from elemental chemostratigraphy, stable isotope chemostratigraphy and magnetostratigraphy are integrated to generate sub-seismic, chronostratigraphic correlations that both compliment and supplement the previously developed platform-to-slope stratigraphic interpretations. Visean, Serpukhovian, and Bashkirian portions of three wells from the inner platform through the platform margin (rim) and into the upper-middle slope were selected for the study. Magnetostratigraphy is carried out in cored parts of study wells and establishes time planes matching the obtained polarity pattern to the Geomagnetic Polarity Timescale (GPTS). The resultant magnetostratigraphy provides absolute age context as well as a coarse framework that must be honoured by any elemental and isotopic chemostratigraphic correlations. The core-based stratigraphic interpretations are then extended into non-cored intervals using elemental and isotopic data derived from cuttings samples. After multiple iterations, correlation solutions are found that maximise the amount of stratigraphic constraints that are honoured, thereby defining a composite, integrated chronostratigraphy for the well transect. The identified trends and concepts can be used to correlate away from the study transect through non-cored wells, and provides modifications to the pre-existing stratigraphic interpretations that can be applied across the field.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013