Arthur D. Donovan

Sequence stratigraphic control on Middle and Upper Jurassic reservoir distribution within the UK Central North Sea

An integrated sequence stratigraphic analysis, utilizing seismic data, well-log cross-sections, biostratigraphic information, and core descriptions, provides a chronostratigraphic framework to analyse and predict the distribution of reservoir sandstones within the Middle to Upper Jurassic section of the UK Central North Sea. The Callovian (Middle Jurassic) through Ryazanian (lowermost Cretaceous) section in the UK Central North Sea forms a depositional wedge that thins towards the western graben margin. This wedge is bounded by surfaces traditionally interpreted as the Middle Cimmerian and Late Cimmerian unconformities. Marginal marine (Fulmar) sandstones are present within this wedge, and can be subdivided into informal lower and upper Fulmar members. Each member has a unique spatial and temporal distribution within the basin that can be related to primary depositional palaeogeography, post-depositional erosion, and basin tectonics. The lower member of the Fulmar Formation is Callovian to Early Oxfordian in age, and is restricted to the Central Graben. This member overlies Middle Jurassic strata of the Pentland Formation, and is conformably overlain by Oxfordian-age strata of the Kimmeridge Clay. The upper member of the Fulmar Formation is Early Kimmeridgian in age, and is distributed along the western graben margin. The upper member is conformably overlain by Kimmeridgian-age strata of Kimmeridge Clay, and typically overlies Triassic deposits of the Skagerrak Formation. Post-depositional erosion, association with relative falls in sea-level and accentuated by basement and salt tectonism, is the major factor that modifies primary depositional patterns within the Fulmar Formation. Truncation associated with unconformities interpreted as the 142 Ma, 138 Ma, and 136 Ma sequence boundaries affect the distribution, fades patterns, and thickness of the upper member of the Fulmar. These unconformities, as well as the 155.5 Ma, 150.5 Ma and 144 Ma sequence boundaries, may also affect the lower member of the Fulmar.

A 3-D Outcrop Perspective of an Unconventional Carbonate Mudstone Reservoir

Eagle Ford outcrops in Lozier Canyon, located in eastern Terrell County in West Texas, provide the opportunity to examine an unconventional carbonate mudstone reservoir at various scales across an area extending over 50 square miles. In Lozier Canyon and its tributaries, tens of cutbank outcrops provide panoramic views of the entire Eagle Ford Group in exposures hundreds of feet high and thousands of feet long. Furthermore, some localities display laterally extensive exposures of individual bedding planes and key stratal surfaces to allow for the 3-dimensional view of sedimentary structures, biogenic debris beds, and trace fossils. Whereas unconventional mudstone reservoirs like the Eagle Ford are commonly portrayed as being homogenous reservoirs, our work to date reveals a vertically heterogeneous facies and TOC succession with variability at the bed-, bedset-, parasequence-, sequence-, and sequence-set scale. Within the study area, lateral variations in the thickness, lithofacies, and bedding continuity within the various defined members and sequences is observed. The Eagle Ford outcrops in West Texas offer a unique 3-D perspective to document, understand, and predict the vertical and lateral heterogeneities that occur at a variety of scales within unconventional mudstones reservoirs, like the Eagle Ford in the subsurface of South Texas, and similar reservoirs elsewhere.

Making outcrops relevant for an unconventional source rock play: an example from the Eagle Ford Group of Texas

Abstract The utility of outcrops to geoscientists working the subsurface is typically limited by provincial nomenclature, outcrop-based lithostratigraphic subdivisions, which commonly differ in name and numbers among researchers, and often a basic indifference to the subsurface stratigraphy. In the subsurface of South Texas, the Eagle Ford Group is commonly divided into an organic-rich Lower Eagle Formation and a carbonate-rich Upper Eagle Ford Formation. In contrast, coeval strata that crop out nearby in West Texas are traditionally referred to as the Boquillas Formation. Adding further complexity to the outcrop stratigraphy is that previous workers divided the Boquillas into two to five informal lithostratigraphic units whose names and boundaries differ among researchers. By convolving information from a handheld gamma ray spectrometer, outcrop geochemical data and sequence stratigraphy, the Lower and Upper Eagle formations, as defined in the subsurface of South Texas, can now be defined in the coeval Eagle Ford Group outcrops of West Texas. Integrating basic geochemical and petrophysical data to the updated stratigraphy on these outcrops was transformative. By doing this, these West Texas exposures became portals to examine, as well as explain and predict, the distribution and thickness variations of specific chronostratigraphic units that are critical to the economic success of exploiting the Eagle Group across the unconventional source rock play fairway in South Texas.