USING MICRO-RESISTIVITY IMAGING AND ELEMENTAL ANALYSIS DATA TO IDENTIFY THIN ORGANIC-RICH BEDS IN THE WILLISTON BASIN, NORTH DAKOTA

摘要

Ordovician G. prisca kukersites provide a potentialsource for the Red River oil in the Williston basinof North Dakota; consequently, the identificationof these intervals can be critical when planningcompletions. Although kukersites are veryorganic-rich [total organic content (TOC) up to59% in the literature], identification of these bedsusing standard openhole logs is hindered becausethe kukersites generally occur only as thin laminaeor beds of less than 1-ft thickness. This is wellbelow the vertical resolution of most openholetools. With a vertical resolution of approximately0.10 in., microresistivity imaging tools can detectthin organic-rich beds; however resistivity alonecannot be used to accurately distinguish betweenan organic-rich bed and other resistive beds suchas anhydrite.Published literature suggests algal bloomsassociated with kukersite deposition would haveslowed or stopped carbonate sedimentation;consequently, the first step in the identification ofkukersites is to use microresistivity image data toidentify thin beds or laminae with high resistivityin dolomitic intervals. After these initialcandidates are identified, triple combination data isexamined to rule out lithologies with similarresistivities but strong identifying signals, such asanhydrite, to reduce the number of potentialkukersite candidates to approximately 20 beds. Anelemental analysis tool was used to furtherinvestigate the likelihood the beds identified werekukersite. An Estonian study showed kukersites tocontain approximately 1.5% total sulfur.Therefore, sulfur was used when solving forkerogen with the elemental analysis within the RedRiver Formation. When solving in this manner, thekerogen signals in the elemental analysis werefound to correspond closely with many of thepossible kukersites identified in the image data.Kukersite-rich intervals are likely to be foundwhere the potentially organic-rich thin bedsidentified in both tools overlap. Sidewall corescollected in these intervals appear to have thesame visual characteristics as kukersites describedin published literature; however, the full analysisof these cores have not yet been completed.This case study demonstrates how solutions todifficult problems can be provided by using aninterdisciplinary approach that incorporates allavailable data as well as a fundamental knowledgeof the geologic environment in which the problemexists.