Robert G. Loucks

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.

Carbonate Sequence Stratigraphy

Derived from the 1991 Research Symposium on Carbonate Sequence Stratigraphy, the authors have brought together in one volume a representative sampling of pivotal research in this important topic. Its three sections describe (1) sequence concepts and sedimentologic principles, (2) seismic sequence case studies involving seismic and outcrop interpretations, and (3) examples of high-frequency, meter-scale cycle deposition and stacking patterns.

Mississippian Barnett Shale: Lithofacies and depositional setting of a deep-water shale-gas succession in the Fort Worth Basin, Texas

The Mississippian Barnett Formation of the Fort Worth Basin is a classic shale-gas system in which the rock is the source, reservoir, and seal. Barnett strata were deposited in a deeper water foreland basin that had poor circulation with the open ocean. For most of the basins history, bottom waters were euxinic, preserving organic matter and, thus, creating a rich source rock, along with abundant framboidal pyrite. The Barnett interval comprises a variety of facies but is dominated by fine-grained (clay- to silt-size) particles. Three general lithofacies are recognized on the basis of mineralogy, fabric, biota, and texture: (1) laminated siliceous mudstone; (2) laminated argillaceous lime mudstone (marl); and (3) skeletal, argillaceous lime packstone. Each facies contains abundant pyrite and phosphate (apatite), which are especially common at hardgrounds. Carbonate concretions, a product of early diagenesis, are also common. The entire Barnett biota is composed of debris transported to the basin from the shelf or upper oxygenated slope by hemipelagic mud plumes, dilute turbidites, and debris flows. Biogenic sediment was also sourced from the shallower, better oxygenated water column. Barnett deposition is estimated to have occurred over a 25-m.y. period, and despite the variations in sublithofacies, sedimentation style remained remarkably similar throughout this span of time.

Paleocave Carbonate Reservoirs: Origins, Burial-Depth Modifications, Spatial Complexity, and Reservoir Implications

Paleocave systems form an important class of carbonate reservoirs that are products of near-surface karst processes and later burial compaction and diagenesis. Features and origins of fractures, breccias, and sediment fills associated with paleocave reservoirs have been studied in modern and ancient cave systems. Information about such cave systems is used in this paper to reconstruct the general evolution of paleocave reservoirs and their associated scale, pore networks, and spatial complexities. Spatial complexities in paleocave reservoirs result from near-surface and burial processes. Near-surface processes include dissolutional excavation, clastic sedimentation, chemical precipitation, and localized fracturing, brecciation, and collapse of cave walls and ceilings. Burial processes begin as cave systems subside into the subsurface. Remaining cave passages commonly collapse and early-formed breccia clasts are rebrecciated. Differential compaction of strata around and over collapsed passages produces fractures, crackle breccias, and mosaic breccias. Near-surface and burial processes combine to produce typically complex reservoirs with several scales of heterogeneity. Hydrocarbon reservoirs of paleocave origin are commonly the product of coalesced collapsed-paleocave systems. The coalescing of passages in a cave system into larger, connected porosity zones results from a combination of multiple, cave-forming episodes at composite unconformities and from the collapse of cave systems during burial where surrounding host strata are brecciated and fractured. This combination of processes creates spatially complex reservoirs that can be hundreds to several thousands of meters across, commonly forming large exploration targets. Final size, pore-network types, and spatial complexities of coalesced collapsed-paleocave systems are products of their evolution from near-surface development through burial into the deeper subsurface. The coalesced collapsed-paleocave reservoir hypothesis explains the scale of reservoirs observed and the spatial complexities involved.

Quantifying the origin and geometry of circular sag structures in northern Fort Worth Basin, Texas: Paleocave collapse, pull-apart fault systems, or hydrothermal alteration?

Three-dimensional seismic data reveal numerous subcircular sag structures in the northern Fort Worth Basin. The structures are defined by concentric faults that extend vertically upward 760–1060 m (2500–3500 ft) from the Lower Ordovician Ellenburger Group. The largest structures remained active into the lower Desmoinesian Strawn Group. Criteria are outlined for defining seismically resolvable sag structures, and a detailed quantitative analysis of the geometries of these circular features was undertaken. Results are compared and contrasted, with reviews of subsurface collapse mechanisms and strike-slip processes that are known to produce subsurface circular to subcircular sag geometries in plan view. In this manner, we develop several constraints for differentiating collapse-related sag structures from strike-slip–related sag structures. Qualitative analyses indicate that the geometries observed are strongly analogous to subsurface collapse features where material is removed at depth to create a void, into which the overburden subsequently sags and collapses. Quantitative analyses support the formation of these features by incremental collapse and suprastratal deformation above a linked system of coalesced, collapsed paleocaves within the Ellenburger Group. Observations and criteria presented herein provide valuable information in defining seismically resolvable collapse features worldwide and help distinguish sag features of collapse affinity from those of other origins.

Ground penetrating radar imaging of a collapsed paleocave system in the Ellenburger dolomite, central Texas

Abstract Ground penetrating radar (GPR) can image near-surface features with high (submeter) resolution. A field feasibility test over a collapsed paleocave system in the Lower Ordovician Ellenburger dolomites in central Texas shows the ability to image both large (10s of m) and small (submeter) scale features. Two-dimensional GPR profiles were recorded along the top of quarry walls to image similar features displayed in the walls. GPR data collected over the transition between the dipping, stratified host rock and the brecciated cave fill are interpreted in terms of the curved contact of a cave wall and roof, pebble to boulder (1–50 cm) sized chaotic breccia, and tension fractures paralleling the cave wall. GPR data collected over a zone of megablocks that resulted from massive cave roof collapse outline blocks that range in size from a few meters to more than 6 m. The scales of features in the GPR data lines correspond well with those noted in the adjacent quarry faces. The different carbonate facies produce readily distinguishable GPR facies. This demonstrates potential feasibility for detailed study of carbonate facies and features from GPR data and suggests 3-D surveys as a desirable next step. Potential applications include 3-D characterization of analogs of collapsed paleocave hydrocarbon reservoirs.

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.

Three-dimensional seismic geomorphology and analysis of the Ordovician paleokarst drainage system in the central Tabei Uplift, northern Tarim Basin, western China

High-quality three-dimensional seismic data acquired in the central Tabei Uplift, Tarim Basin, western China, provide a rare opportunity to characterize in exceptional detail the three-dimensional geomorphology of a deeply buried (5500–6500 m [18,045–21,325 ft]) Ordovician unconformity and the related paleokarst drainage system. An integrated approach was applied that emphasized integration of seismic data with available conventional core, wireline logs, and age-equivalent outcrops. The exceptional quality of the seismic data allowed a seismic detection limit of karstified features of less than 75 75 m (246 246 ft) horizontally and 6 m (20 ft) vertically. Interpreted geomorphologic and depositional elements include fluvial channels and canyons, fluvial valleys, sinkholes, and tower karsts and hills. The modern tower karst-drainage system in Guilin, China, is very similar to the mapped Ordovician karst-drainage system and is used as a modern analog. The interaction between the surface karst-drainage system and the shallow-subsurface cave-passage system is evidenced by the observation that surface canyons appear to initiate in areas associated with intense sinkhole development. Also, surface river valleys tend to correspond to dip-oriented surface depressions partly related to near-surface cave collapse. During burial into the deeper subsurface, the combination of intrastratal collapse (karstified strata) and suprastratal collapse (postkarst-deposited strata) created large damage zones hundreds of meters thick and kilometers wide. Coalesced-collapsed paleocave systems can be interpreted from the unique circular pattern of faults (observed in map view) that are associated with seismic bright spots.

History of burial diagenesis determined from isotopic geochemistry, Frio Formation, Brazoria County, Texas

Progressive burial diagenesis of the Oligocene Frio Formation in Brazoria County, Texas, has resulted in extensive reaction between pore fluids and sediment in a major shift toward water/rock equilibrium. Carbon and oxygen isotopic data, combined with fluid isotopic data from the literature, indicate that quartz cement formed at 75 to 80°C and kaolinite at approximately 100°C. The zone of most rapid albitization is near 150°C. Authigenic carbonates formed over a wide range of temperatures, and those within the peak zone of hydrocarbon generation are depleted in 13C. At depths shallower than approximately 2,600 m, quartz and carbonate cementation in primary intergranular pore spaces (passive diagenesis) dominated. Below 2,600 m, within the geopres ured zone, reaction of detrital components (active diagenesis) is the major process. Organic maturation, albitization, and the transition of smectite to illite are the processes that contribute most of the components required for precipitation of cements. Quartz cementation occurred quite early in the burial history of the Frio (beginning at approximately 1,500 m of burial), when rates of fluid expulsion were at a maximum and when little of the Frio sandstone section had reached the zone of albitization.

Three-dimensional architecture of a coalesced, collapsed-paleocave system in the Lower Ordovician Ellenburger Group, central Texas

The three-dimensional, interwell-scale architecture of a Lower Ordovician Ellenburger coalesced, collapsed-paleocave system was constructed through the integration of ground-penetrating radar (GPR), shallow-core, and outcrop data. The data were collected near Marble Falls in central Texas over an area (800 1000 m [2600 3300 ft]) that could cover several oil-well locations (160 ac; 0.65 km2) typical of a region such as west Texas. Integration of core-based facies descriptions with GPR-reflection response identified several paleocave facies that can be recognized and mapped with GPR data alone: (1) continuous reflections image the undisturbed strata, (2) relatively continuous reflections (over tens of meters) characterized by faults and folds image the disturbed strata, and (3) chaotic reflections having little to no perceptible continuity image heterogeneous, cave-related brecciated facies recognized in core that cannot be individually resolved with the GPR data. These latter facies include the highly disturbed strata, coarse-clast chaotic breccia, fine-clast chaotic breccia, and sediment fill.The three-dimensional architecture of the coalesced, collapsed-paleocave system based on core and GPR data indicates that there are trends of brecciated bodies that are as much as 350 m (1100 ft) wide, greater than 1000 m (3300 ft) long, and tens of meters high. These brecciated bodies are coalesced, collapsed paleocaves. Between the brecciated bodies are areas of disturbed and undisturbed host rock that are jointly as much as 200 m (660 ft) wide.As a cave system is buried, many structural features form by mechanical compaction. These features include folds, sags, and faults. The folds and sags measure from a few meters to several hundred meters wide. The collapse-related faults are numerous and can have several meters of displacement. Most are normal faults, but reverse faults also occur.