An assessment of fundamental controls on petrophysical variability of organiccarbon rich mudstones: an integrated wireline log lithofacies study of Jurassic (Toarcian) shales

Fine-grained sedimentary rocks are very common and play an important role as source rocks in conventional hydrocarbon plays and combined source rocks and reservoirs in unconventional shale gas plays. Recent research has revealed that organic carbon-rich examples are much more heterogeneous than most authors have previously assumed. Despite appearing largely homogeneous at hand specimen scales, they are actually organized into very thin, sharp-based, occasionally bioturbated beds that systematically upward-coarsen and fine on metre scales. The presence of this heterogeneity profoundly impacts how geologists interpret both the fundamental mechanisms responsible for organic carbon enrichment in these successions, how long-term stratigraphic processes are interpreted to control facies variability, and suggest that the relative importance of persistent bottom water anoxia and that the existence of a low energy water column have been over estimated. This study is focused on the Posidonia Fm., a Jurassic (Toarcian) mudstone succession located within the Lower Saxony Basin, Germany. The Posidonia Fm. was chosen because this succession, and its global equivalents distributed throughout Europe, South America, and North America, are considered to be a classical example of an Oceanic Anoxic Event (Jenkyns, 1988). Additionally, the availability of two Posidonia Fm. cores with correlative wire line log suites allowed for an integrated lithofacies-wireline log assessment. The organic-carbon rich lithofacies present in the Posidonia Formation was investigated to determine (a) the extent of lithofacies variability present, (b) the fundamental physical, chemical and biological controls that might be responsible for this variability, (c) whether this succession can be reasonably interpreted in terms of sequence stratigraphic principles, and (d) the appropriateness of using wireline logs to characterize this variability. The necessary textural and mineralogical data to investigate these aims were obtained using petrographic, scanning electron microscope, x-ray diffraction data, and total organic carbon data coupled with a suite of wire line logs (gamma ray, neutron porosity, sonic, density, and resistivity logs). Seven lithofacies were identified and are characterized by varying proportions of detrital and production-derived rock components, diagenetic features, grain size variations and variable textures and bedding fabrics. These lithofacies are organized into thin mm-scale beds that systematically stack into m-units that are variously increasingly silt-rich, clay-rich or calcareous nannoplankton-rich. The deposition of normally-graded beds with sharp bases are interpreted to be related to storm deposition, and evidence of ripple deposition suggests that depositional conditions were more energetic than previously assumed. Sand-rich ripples suggest that advective transport mechanisms were operating in addition to suspension settling processes. Stacked successions progressively enriched (or depleted) in detrital components relative to production-derived components and exhibiting evidence of increasing (or decreasing) energy conditions have been determined to be component parts of overall highstand ( and transgressive) systems tracts. Stacked successions characterized by systematic facies variability correlate to increasing and decreasing trends in the gamma-ray, density, sonic, neutron, and resistivity wireline logs. The sensitivity of the logs to mineralogy means that systematic lithological variations can be related to wireline trends. Cross-plots of neutron, sonic, density and resistivity log data appear to broadly discriminate between facies. A single log cannot be relied upon to extract useful facies information, an integrated approach using two or more logs is much more effective