Improved Wellbore Positioning Using Rigsite Elemental Chemostratigraphy in the Tyonek Fluvial Sequence, Cosmopolitan Field, Cook Inlet, Alaska
Even when utilizing the most advanced LWD tools, maintaining stratigraphic position and accurate lithologic perspective in a fluvial sequence can be challenging. In the case described here, a sinusoidal wellbore was drilled in a 6,400 feet exploration lateral to evaluate an 85-foot thick sandstone target zone in Cosmopolitan Field. The wellbore attempted to follow the target interval over the crest of an anticlinal structure, and down the opposite side.
In order to meet this positioning challenge, chemostratigraphic correlation was performed at the wellsite while drilling using portable ED-XRF technology. Prior to drilling, a laboratory study was conducted on cuttings from several existing offset wells through the target zone. Whole-rock geochemical data for about 45 elements was collected from high-precision ICP spectroscopy instruments. This data was used to construct a chemostratigraphic zonation that allowed better resolution of stratigraphic units within the clean fluvial sandstones than is possible with traditional technologies. The results of this study were used to custom-calibrate the wellsite ED-XRF instrument prior to drilling, and elemental values could then be measured directly at wellsite from fresh cuttings.
The target interval was divided into four chemostratigraphic units which that could be discriminated by the elemental data. Key ratios were identified to accomplish this discrimination: Cs/Al, K/Al, Nb/Al, Fe/Al, Th/Cs, Zr/Nb, Na/Al, Ti/Al, and Rb/Al. These ratios appear to be driven by changes in feldspars, micas, and heavy minerals through the clean sandstone sections. Some siltstones, mudstones, and coals were also encountered in overbank facies and could be recognized. The key ratios were monitored while drilling to ascertain stratigraphic position within the Neogene fluvial sequence. The chemostratigraphic data allowed for the near real-time recognition of units as thin as four feet (TVD) while drilling, as well as the recognition of faults along the path of the wellbore.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009