1. Department of Geology and Geological Engineering Van Tuyl Lecture Series-Weimer Distinguished Lecture in Sedimentary Geology - Fall 2015 4:00-5:00 p.m. in Berthoud Hall Room 241 Thursday, December 3, 2015 Peter Scholle Retired NM State Geologist Scholle Petrographic LLC “Fifty-one Shades of Gray: The Deposition and Diagenesis of North Sea Chalks” Abstract: North Sea (NS) chalks make an interesting comparison with rocks of similar lithology and age in the Western Interior (WI) Basin (especially the chalk facies of the Niobrara Formation). There are some differences between the two chalk deposits: typically NS chalks are very clean limestones (96-99% CaCO 3 ) and have very low TOC levels (rarely above 0.2%), whereas WI chalks can have very high clay contents (at least 5% and commonly far higher) and TOC levels (in the 0.5-6% range) making WI chalks self-sourcing. European chalks were deposited over a much wider area but under generally comparable paleogeographic circumstances, in a broad epicontinental shelf sea. The European setting, however, had more normal salinity and little of the water-mass stratification seen in the WI Basin. A further major difference in setting is the more active synsedimentary halokinesis and inversion tectonics in Europe, especially in the NS region. The active NS halokinesis and tectonics played a major role in large-scale chalk resedimentation, enormous variations in unit thickness and localization of hydrocarbon reservoirs. Locally extremely high rates of chalk accumulation (largely a result of resedimentation) as well as high rates of post-chalk shale deposition in the Cenozoic, led to regional overpressuring which played an extremely important role in the diagenetic history, the most important controlling factor in most NS chalk reservoirs. The basic principles of such chalk diagenesis are well understood—the primary drivers of porosity loss are burial-related pressure and temperature increases and the major inhibitions on such losses are overpressuring and early hydrocarbon entry. Although those factors apply equally to WI and NS chalks, the above-mentioned conditions in the NS led to early and widespread overpressuring that persists to this day and has allowed development of numerous giant and even supergiant oil and gas fields producing from thick pay sections in highly overpressured chalks with porosities as high as 40-50% at depths of 3-4km. Such overpressuring does exist in parts of the WI region, but mainly in areas where the Niobrara chalk has transitioned to shale. On the other hand, NS chalks have no laterally equivalent shale facies that can act as source rocks and/or unconventional reservoirs. The substantial bed-to-bed porosity variations seen in many NS and other chalks, variations that are especially important for horizontal drilling, require different, smaller-scale explanations. Variations in clay content, amounts of seafloor cementation and sediment reworking (especially debris flows) can explain some, but not all bed-scale porosity variations in chalks. Considering the fact that coccolithophores are the dominant constituents of most chalks, and despite the fact that they have been shown to have protective polysaccharide coatings, it is coccolith dissolution and/or overgrowth development that is the major factor in chalk porosity variability at bed and smaller scales. That dissolution and/or cementation varies with species composition and diversity and thus initial species makeup of chalks is of great significance to ultimate porosity. However, such primary compositional variations are hard to decipher after substantial diagenetic alteration.