Ursula Hammes

Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores

Matrix-related pore networks in mudrocks are composed of nanometer- to micrometer-size pores. In shale-gas systems, these pores, along with natural fractures, form the flow-path (permeability) network that allows flow of gas from the mudrock to induced fractures during production. A pore classification consisting of three major matrix-related pore types is presented that can be used to quantify matrix-related pores and relate them to pore networks. Two pore types are associated with the mineral matrix; the third pore type is associated with organic matter (OM). Fracture pores are not controlled by individual matrix particles and are not part of this classification. Pores associated with mineral particles can be subdivided into interparticle (interP) pores that are found between particles and crystals and intraparticle (intraP) pores that are located within particles. Organic-matter pores are intraP pores located within OM. Interparticle mineral pores have a higher probability of being part of an effective pore network than intraP mineral pores because they are more likely to be interconnected. Although they are intraP, OM pores are also likely to be part of an interconnected network because of the interconnectivity of OM particles. In unlithifed near-surface muds, pores consist of interP and intraP pores, and as the muds are buried, they compact and lithify. During the compaction process, a large number of interP and intraP pores are destroyed, especially in ductile grain-rich muds. Compaction can decrease the pore volume up to 88% by several kilometers of burial. At the onset of hydrocarbon thermal maturation, OM pores are created in kerogen. At depth, dissolution of chemically unstable particles can create additional moldic intraP pores.

Understanding growth-faulted, intraslope subbasins by applying sequence-stratigraphic principles: Examples from the south Texas Oligocene Frio Formation

A detailed analysis of Oligocene Frio Formation intraslope, growth-faulted subbasins in the Corpus Christi, Texas, area indicates that deposition during relative lowstands of sea level was the main initiator, or trigger, of growth faulting. Lowstand depocenters on the low-gradient, upper continental slope comprising basin-floor fan facies, slope-fan systems, and prograding lowstand delta systems exerted sufficient gravity stress to trigger major sections of outer shelf and upper slope strata to fail and move basinward. The faults sole out deep in the basin, and rotation of hanging-wall blocks mobilized deep-water muds and forced the mud basinward and upward to form mud (shale) ridges that constitute the basinward flank of intraslope subbasins overlying footwall fault blocks.Sedimentation associated with third-order relative falls of sea level produced load stress that triggered a major regional syndepositional growth-fault system. Subbasins on the downthrown side of each arcuate fault segment that constitute a regional fault system are filled during the lowstands of sea level. Consequently, genetically similar but noncontemporaneous lowstand depositional systems filled each successive growth-faulted subbasin trend. The subbasin stratigraphy becomes younger basinward because the subbasin development and fill process extended the Frio shelf edge stepwise into the Oligocene Gulf of Mexico Basin, coinciding with relative third-order sea level cycles.The subbasins have been prolific petroleum targets for decades and are now the focus of prospecting for deep gas. Lowstand sandstones are principal reservoirs, and synsedimentary tectonics produced anticlinal and fault traps and associated stratigraphic pinch-out traps on the flanks of the structures. Understanding the origin of the faulted subbasins and their chronostratigraphic relationships and depositional processes provides a perspective that can improve deep gas exploration.

Cathodoluminescence in Calcite Cements: New Insights on Pb and Zn Sensitizing, Mn Activation, and Fe Quenching at Low Trace-Element Concentrations

ABSTRACT The role of Pb and Zn as sensitizers of cathodolumines (CL) in sedimentary carbonates is still speculative, as is the relationship between Mn, Fe, and CL intensity at low concentrations of Mn and Fe. We address these questions using calcite cements from the Lower Oligocene Suwannee Formation of Florida, which have mean Mn, Pb, Zn, and Fe concentrations less than 45 ppm. Only a small fraction of these cements exhibit visible CL, but when present, the luminescence is orange to reddish orange. Trace-element contents were measured using a synchrotron x-ray fluorescence microprobe, and CL intensity was approximated by spot-metered photomicrograph exposure times. Based on Mn and Fe contents, four cement populations can be defined. Group I is luminescent calcite with Mn > 25 ppm and molar Mn/Fe > 1.8. Group II contains both luminescent and nonluminescent cements, all with Mn > 25 ppm, molar Mn/Fe 25 ppm, molar Mn/Fe 100 ppm. Group IV is nonluminescent calcite with Mn < 25 ppm and variable Fe. These groupings and statistical analyses of the data (Mann-Whitney tests and factor analyses) confirm that 25 ppm Mn is necessary to initiate visible CL, and that Mn is the sole control on CL intensity when Mn and Fe contents are very low (<100 ppm). The data also establish 100 ppm Fe as a new minimum for the quenching of CL when Mn is below 200 ppm. There is no evidence in our data that Pb or Zn concentrations in tens of parts per million either activate or sensitize CL. The Mann-Whitney test shows that there is no statistical evidence at the 95% confidence level to indicate that luminescent and nonluminescent cements reflect different populations of Pb and Zn, except for Group II cements that have more Pb in nonluminescent cements than in otherwise chemically equivalent luminescent cements. Factor analysis shows that CL intensity in all luminescent cements is unrelated to Pb or Zn concentrations. Given that Pb or Zn sensitizing is lacking in low-Mn calcites like the Suwannee cements, it is hard to imagine how these elements could play a significant role in calcites containing even more Mn, as is typical of most sedimentary carbonates.

Calcite cementation in the upper Floridan aquifer: A modern example for confined-aquifer cementation models?

Detailed petrographic study of the Oligocene Suwannee limestone (a principal unit of the upper Floridan aquifer system) indicates that all the calcite cements in the Suwannee limestone are precompaction in origin. Most are clearly associated with either early mineralogical stabilization or Oligocene subaerial exposure features. None of the cements are in isotopic (δ 18 O and δ 13 C) or trace element (Fe, Mn, and Sr) equilibrium with the extant aquifer waters. The cements therefore formed prior to the onset of regionally confined, freshwater aquifer conditions. These rocks have been basically inert with respect to calcite cementation during the past 5 to 10 m.y. while they have been part of the regional confined aquifer. Using this modern aquifer as an example, it is likely that confined paleoaquifer cementation has been overestimated as an explanation for regional cementation patterns in ancient limestones.

Unconventional reservoir potential of the upper Permian Zechstein Group: a slope to basin sequence stratigraphic and sedimentological evaluation of carbonates and organic-rich mudrocks, Northern Germany

The Late Permian Zechstein Group in northeastern Germany is characterized by shelf and slope carbonates that rimmed a basin extending from eastern England through the Netherlands and Germany to Poland. Conventional reservoirs are found in grainstones rimming islands created by pre-existing paleohighs and platform-rimming shoals that compose steep margins in the north and ramp deposits in the southern part. The slope and basin deposits are characterized by debris flows and organic-rich mudstones. Lagoonal and basinal evaporites formed the seal for these carbonate and underlying sandstone reservoirs. The objective of this investigation is to evaluate potential unconventional reservoirs in organic-rich, fine-grained and/or tight mudrocks in slope and basin as well as platform carbonates occurring in this stratigraphic interval. Therefore, a comprehensive study was conducted that included sedimentology, sequence stratigraphy, petrography, and geochemistry. Sequence stratigraphic correlations from shelf to basin are crucial in establishing a framework that allows correlation of potential productive facies in fine-grained, organic-rich basinal siliceous and calcareous mudstones or interfingering tight carbonates and siltstones, ranging from the lagoon, to slope to basin, which might be candidates for forming an unconventional reservoir. Most organic-rich shales worldwide are associated with eustatic transgressions. The basal Zechstein cycles, Z1 and Z2, contain organic-rich siliceous and calcareous mudstones and carbonates that form major transgressive deposits in the basin. Maturities range from over–mature (gas) in the basin to oil-generation on the slope with variable TOC contents. This sequence stratigraphic and sedimentologic evaluation of the transgressive facies in the Z1 and Z2 assesses the potential for shale-gas/oil and hybrid unconventional plays. Potential unconventional reservoirs might be explored in laminated organic-rich mudstones within the oil window along the northern and southern slopes of the basin. Although the Zechstein Z1 and Z2 cycles might have limited shale-gas potential because of low thickness and deep burial depth to be economic at this point, unconventional reservoir opportunities that include hybrid and shale-oil potential are possible in the study area.

Seismic interpretation of mass-transport deposits within the upper Oligocene Frio Formation, south Texas Gulf Coast

We integrated well logs and three-dimensional seismic data to describe a wedge of deformed shallow Frio rocks lying above a major bed-parallel decollement within the upper Oligocene Frio Formation located between the Houston and Norias deltas on the south Texas Gulf Coast. Our analyses show that the identified deformed shallow Frio rocks can be divided into proximal clay-rich and low-permeability sandstones characterized by discontinuous, mounded, and chaotic seismic events; near-proximal, clay-poor, and high-permeability sandstones characterized by parallel to subparallel bedded seismic events; and distal sand-, silt-, and mudstones composed of a mix of proximal and near-proximal rocks. All of the deformed rocks are composed of acoustic-impedance materials that are lower than those of the undeformed shallow Frio and are underlain by low-velocity, overpressured, shale-rich rocks. The mechanism that triggered the collapse of the shallow Frio and subsequent development of mass-transport deposits is attributed to an uprising, overpressured, shale-rich high and the development of a shelf-edge listric fault. Proprietary biostratigraphic data show that the collapse of the shallow Frio in areas between the Houston and Norias deltas occurred between 27.5 and 25.3 Ma—approximately the same time as the Hackberry collapse in the Mississippi delta. In the proximal area, interpreted paleowater depths from biostratigraphic data based on benthic foraminifers range from 60 to 120 ft (20–40 m) in a shallow neritic environment. In contrast, the distal area lies in paleowater depths interpreted to be between 120 and 300 ft (40–90 m) in a middle neritic environment.

Regional assessment of the Eagle Ford Group of South Texas, USA: Insights from lithology, pore volume, water saturation, organic richness, and productivity correlations

AbstractA comprehensive regional investigation of the Eagle Ford Shale linking productivity to porosity-thickness (PHIH), lithology (Vclay), pore volume (PHIT), organic matter (TOC), and water-saturation (SW) variations has not been presented to date. Therefore, isopach maps across the Eagle Ford Shale play west of the San Marcos Arch were constructed using thickness and log-calculated attributes such as TOC, Vclay, SW, and porosity to identify sweet spots and spatial distribution of these geologic characteristics that influence productivity in shale plays. The Upper Cretaceous Eagle Ford Shale in South Texas is an organic-rich, calcareous mudrock deposited during a second-order transgression of global sea level on a carbonate-dominated shelf updip from the older Sligo and Edwards (Stuart City) reef margins. Lithology and organic-matter deposition were controlled by fluvial input from the Woodbine delta in the northeast, upwelling along the Cretaceous shelf edge, and volcanic and clastic input from distan...

Linear amplitude patterns in Corpus Christi Bay Frio Subbasin, south Texas: Interpretive pitfalls or depositional features?

Linear amplitude patterns on stratal slices in the Corpus Christi Bay area of Texas are important seismic geomorphological features that reflect sediment dispersal patterns. These amplitude patterns are oriented in both strike and dip directions. Some of the linear amplitude patterns are related to faults; however, most are related to orientation of sandstone bodies. Faulting may make the depositionally related linear amplitude patterns more fragmented, but faults do not destroy the overall orientation or geomorphologic significance of these patterns. Amplitude patterns on stratal slices should be interpreted as unbiased, general, sediment-dispersal patterns unless the patterns can be tied directly to a structural feature. In-depth understanding of structural and depositionally related amplitude patterns leads to more accurate stratal slicing interpretation in facies/reservoir prediction using poststack 3D seismic data.

Linear amplitude patterns in Corpus Christi Bay Frio Subbasin, South Texas: Artifacts or depositional features?

Linear amplitude patterns on stratal slices in the Corpus Christi Bay area of Texas are important seismic geomorphological features that show sediment dispersal patterns. These amplitude patterns are oriented in both strike and dip directions. Some of the linear amplitude patterns are related to faults; however, most of them are not associated with faults but are related to sandstone bodies. Later faulting may make the depositional-related linear amplitude patterns more fragmented, but the faults will not change the overall orientation or the geomorphologic significance of the depositional-related linear amplitude patterns. In some cases the depositional-related linear amplitude patterns parallel the long axes of faults, which may have has come control on the depositional pattern. Amplitude patterns on stratal slices should be interpreted as non-biased, general sediments dispersal patterns unless the patterns can be directly ties to a structural feature. Indepth understanding of structural and depositional-related amplitude patterns leads to accurate stratal slicing interpretations. This is important because stratal slicing is a low cost and useful tool for facies/reservoir prediction with poststack 3-D seismic data.