The Late Cretaceous Epoch was characterized by major global perturbations in the carbon cycle, the most prominent occurring near the Cenomanian–Turonian (CT) transition marked by Oceanic Anoxic Event 2 (OAE-2) at 94.9–93.7 Ma. The Cretaceous Western Interior Seaway (KWIS) was one of several epicontinental seas in which a complex water-mass evolution was recorded in widespread sedimentary successions. This contribution integrates new data on the main components of organic matter, geochemistry, and stable isotopes along a north–south transect from the KWIS to the equatorial western Atlantic and Southern Ocean. In particular, cored sedimentary rocks from the Eagle Ford Group of west Texas (∼ 90–98 Ma) demonstrate subtle temporal and spatial variations in palaeoenvironmental conditions and provide an important geographic constraint for interpreting water-mass evolution. High-latitude (boreal–austral), equatorial Atlantic Tethyan and locally sourced Western Interior Seaway water masses are distinguished by distinct palynological assemblages and geochemical signatures. The northward migration of an equatorial Atlantic Tethyan water mass into the KWIS occurred during the early–middle Cenomanian (98–95 Ma) followed by a major re-organization during the latest Cenomanian–Turonian (95–94 Ma) as a full connection with a northerly boreal water mass was established during peak transgression. This oceanographic change promoted de-stratification of the water column and improved oxygenation throughout the KWIS and as far south as the Demerara Rise off Suriname. In addition, the recorded decline in redox-sensitive trace metals during the onset of OAE-2 likely reflects a genuine oxygenation event related to open water-mass exchange and may have been complicated by variable contribution of organic matter from different sources (e.g. refractory/terrigenous material), requiring further investigation.

Magnetic Susceptibility (MS) is an indicator of the concentration of magnetic particles in rocks. In pre-Quaternary sediments the magnetic susceptibility is often sourced in either Fe-rich clays (chlorites etc), or Fe-oxides (magnetite or hematite), and often shows a dilution-relationship with calcite which has a small negative MS. Mudrocks lend themselves readily to MS analyses, since MS often responds to the gross lithological variations, with a superimposed provenance or sometimes diagenetic signature. Two applications for magnetic susceptibility in shale resource plays will be considered in this paper: a) stratigraphic correlation and b) paleoflow determination. The first is carried out using data acquired from either small samples, measured in the laboratory, or by direct analysis of cores using a hand held MS meter. Paleoflow determinations utilise directional variation in magnetic susceptibility (Anisotropy of Magnetic SusceptibilityAMS) to make an interpretation of grain orientation. Re-orientation of the core is required to convert the preferred grain orientation into geographic coordinates. Direct measurement of core using a handheld magnetic susceptibility meter enables large, high resolution (5-10 cm spacing) datasets to be gathered rapidly. Typically, these data show marked cyclicity, which in a Miocene carbonate sequence from Mallorca will be shown to be controlled by sea level fluctuations. Furthermore, because high resolution measurements are available, parasequences can be imaged in the magnetic susceptibility data. Changes in the symmetry of the transgressive regressive portions of parasequences allow variations in “stacking patterns” to be compiled, thereby providing input into sequence stratigraphic interpretations. This aspect will be demonstrated using core analysis from a US shale play. AMS measurements provide a rapid and precise determination of the three-dimensional orientation of grains in samples. When dealing with a shale play, any such grain-orientation data are difficult to determine using visual analyses. The AMS expresses the bedding-foliation, and the lineation (i.e. paleoflow direction) within the bedding plane. Hence, it can be used to infer structural information, as well as with-bedding preferred grain-orientation information. Here, we will show initial results from a European shale play that suggests AMS has the potential to be a powerful tool in paleoflow and sediment fabric analysis of mudrocks.

Abstract The southeastern Sichuan Basin is potentially one of the most prolific Shale Gas play in China. There is an obvious and pervasive stratigraphic correlation (high Gamma reading) at the lower Longmaxi Formation, which has been defined as the Hot Shale interval. However, it is difficult to make a meaningful detailed geological correlation within the Hot Shale interval due to relative poor data coverage of seismic and petrophysical log signatures. Chemostratigraphy under the right geological setting is a proven tool that uses changes in bulk elemental composition to understand controls on reservoir quality, such as sediment provenance (proximal or distal) and paleo-environment in order to establish a chemostsratigraphic correlation for a given interval. For this study, a total of 377 cuttings and 20 core samples from the Upper Longmaxi, Lower Longmaxi to Wufeng (Hot Shale) and Linxiang/Baota formations of 4 exploration wells have been analyzed using ICP-OES and ICP-MS. Final integrated analyses have been made by combining with petrophysical logs, TOC, X-Ray Diffraction (XRD), petrography and Scanning Electron Microscope (SEM) data. Prior to any interpretation, cutting sample quality was checked by comparison between chemical gamma (using U, K and Th data from the sample) and downhole gamma which suggested the cutting samples were largely representative of the formation drilled. Elements and element ratios can be directly linked to the XRD data which reflects the influence of sediment lithology/ mineralogy variation, it also demonstrates the elemental data is qualified to be used. The enrichment factor of vanadium (EFV), which is sensitive to changing paleo-environments, suggests the lower Hot Shale was deposited in an overall more anoxic environment compared to the overlying and underlying formations. Within the Hot Shale interval, there is a positive relationship between TOC and EFV, indicating that TOC preservation is predominantly controlled by changes in paleo-environmental settings in the southeastern Sichuan basin. It further implies that EFV can also be used as a proxy for modeling TOC to assist in predicting lateral changes of paleo-environment between widely spaced data points. Changes in the relative amount of terrigenous input (TiO2 + Al2O3 + K2O + Na2O) and biogenetic silica (SiO2/Zr) are also modeled, the former being related to changes in sediment provenance and the latter to depositional conditions. Terrigenous input increases upward with the lower Hot Shale having lower terrigenous content indicating a period of sediment starvation in relatively deep water. The high biogenic silica contents in the lower Hot Shale interval are consistent with the pervasive presence of sponge spicules in the microscope view of thin sections. The high biogenetic silica contents results in the sediments being more brittle, thus favorable for Hydraulic Frac Stimulation.

Chemostratigraphy uses geochemical data to characterise and correlate sedimentary strata. The value of the technique has been recognised as long ago as the 1930’s, but its application was limited up until relatively recently due to the lack of accurate, precise and rapid turnaround analytical instruments. With the development of modern instrumentation and data processing technology it is now possible to rapidly quantify over 40 elements in a range of different sample types including cores and cuttings. These posters illustrate the techniques involved in producing elemental data and the capabilities of the technology. Two case studies are then utilised to show how the concept of chemostratigraphy is applied in real geological settings.

The Mungaroo Formation in the Gorgon Field is a stratigraphically complex fluvial system of Triassic age. It is also a major hydrocarbon reservoir, therefore understanding its internal stratigraphic architecture is of paramount importance to exploitation of its reserves. Here, the technique of chemostratigraphy is used to construct a correlation framework for the Mungaroo Formation of the Gorgon Field. Chemostratigraphy is a tool that employs variations in inorganic whole rock geochemistry to enable the characterisation and subsequent correlation of sediments. For this study, a total of 1,514 cuttings and core samples from eight wells in the Gorgon Field have been analysed. Using data derived from both claystone and sandstone lithologies, the Mungaroo Formation is divided into nine chemostratigraphic packages, 22 geochemical units and 19 sand units. Additionally, three surfaces identified as time lines (T1–T3) are geochemically defined. Changes in values of Ga/Rb and Al2O3/(CaO+ MgO+K2O+Na2O) indicate that during deposition of the Mungaroo Formation, the paleoclimate became warmer and wetter, resulting in increasingly intense hydrolytic weathering. Steps in the values of these ratios allow three surfaces to be identified (T1–T3), at which there is a marked and sustained change in the paleoclimate. These three surfaces represent time lines that provide a quasi-chronostratigraphic framework for the formation. Values of Cr/Al2O3, Cr/Na2O and Nb/Al2O3 are related to changes in sediment provenance and indicate that during deposition of the Mungaroo Formation the provenance became more mafic and less intermediate. It is variations in paleoclimate and provenance modelled from the geochemical data that allows the packages, units and sand units to be characterised and correlated. The chemostratigraphic correlation is more detailed than is available from other stratigraphic techniques. Although in most instances the lithostratigraphic correlation of sand units based on wireline log correlation matches the one defined using chemostratigraphy, there are some significant differences between the two that influence reservoir models and gas production.

The Upper Cretaceous Cenomanian/Turonian (C/T) boundary represents the most significant thermal climatic event in the Mesozoic, and it has been well-studied. Here we present a relatively new approach for providing both independent, climate-based cyclostratigraphic analysis and correlation among C/T sections. We have collected samples for bulk (initial) low-field magnetic susceptibility (MS) measurement and high-resolution inorganic geochemical analyses from two C/T sequences: (1) the well-defined, C/T Global boundary Stratotype Section and Point (GSSP) located at Lake Pueblo, Colorado, and deposited in the Upper Cretaceous Western Interior Seaway of North America, and (2) the well-studied marlstone and limestone USGS#1 Portland Core, drilled at Portland, Colorado, ∼40 km to the west of the C/T GSSP. Comparing the lithostratigraphy, chemostratigraphy, and magnetostratigraphic susceptibility (developed from magnetic susceptibility (MS) data) indicates that the USGS#1 Portland Core closely compares with the C/T GSSP sequence using any of the three measures, demonstrating that intervals collected and sampled from these two sequences are highly correlated. A preliminary magnetostratigraphic susceptibility zonation for the C/T boundary is presented to which other MS work can be compared. Fourier Transform (FT) analysis of the MS data from the GSSP, when compared with previous FT work using geochemical analysis of Portland core samples shows close similarities to the GSSP, with Milankovitch eccentricity, obliquity and precession bands well-defined in both data sets. Using this result, we assign relative ages to the intervals sampled and evaluate changes in absolute sediment accumulation rates through the C/T boundary interval. We show that while sediment accumulation rate is relatively high in the upper Cenomanian, a change toward lower accumulation rates in the GSSP section at the C/T boundary is due to reduced productivity following the C/T extinction events that resulted from global warming associated with Oceanic Anoxic Event 2 (OAE2) that began in the uppermost Cenomanian.