Print this pageA Simple Solution for Resolving Complex Mineralogy in Pri-Caspian Basin Lower Carboniferous Reef Carbonates - Karachaganak Field, Kazakhstan
The Karachaganak gas condensate field reservoir is a carbonate reef, located in the Pricaspian Basin, north-western Kazakhstan. The diagenetic history of the Lower Carboniferous (Serpukhovian - Visean) limestones is complex and unpredictable. The operator needed a reliable method of estimating percentages of limestone, anhydrite and more particularly, diagenetic dolomite in the reservoir, without recourse to cutting core in every well. Petrophysical multi-mineral calculations have been used, but at times seem to produce inconsistent results, at variance with observation of cuttings and even calcimetry. Using core plugs was not feasible, due to infrequent and incomplete cores, sample spacing and selection bias. X-ray diffraction (XRD) analysis of high resolution cuttings samples, instead of core plugs, was attempted as a more statistically reliable mineralogy estimate, for comparison with petrophysical calculations.
High resolution drilled cuttings samples in a development well were caught at 1m intervals or less and analyzed for minerals common to carbonate environments, primarily calcite, dolomite and anhydrite. A western laboratory was initially used as a control, but sending samples out of the country was expensive and sample turn around too long. A local laboratory in Kazakhstan offered analysis over the same interval and their results were consistent with the control sample set and with petrophysical calculations. In addition they offered an economic and timely solution. Core photographs across one of the intervals were also analyzed using a point count technique to determine anhydrite content. Petrophysical multi-mineral calculations were made over the same interval.
Results from XRD of the cuttings samples correlated very well with calcite-dolomite mineralogy calculated from petrophysical multi-mineral analyses. The point count technique was also deemed successful and confirmed the peaks seen on petrophysical analysis although both visual and XRD techniques suggested petrophysics slightly over-estimated anhydrite content. A second pilot study demonstrated a connection between dolomite occurrence and anhydrite.
XRD analysis of cuttings samples is encouraging and will be continued for upcoming wells and has already been used for verification and back-up for petrophysics. The results will contribute to refining petrophysical multi-mineral model analyses, which will then be incorporated into a new geological model of Karachaganak.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009