Interior Facies Research Articles

Reconstructing depositional environments through cave interior facies: The case of Galería Complex (Sierra de Atapuerca, Spain)

This work showed that cave sediments are useful for geomorphologic studies and for reconstructing depositional environments. While the cave entrance facies have been extensively studied for their relationship with the fossil and archaeological record, the cave interior facies have received much less attention, although they can provide much information on the geomorphological evolution of the karst. This work presents the stratigraphic and sedimentological study of a section >6 m thick and 10 m long of cave interior sediments of Galería Complex (Sierra de Atapuerca, Spain). Galería Complex is a cavity infill of the Sierra de Atapuerca (Spain), composed of three sections filled by at least 30 m of Pleistocene sediments. This sequence is divided into 5 lithostratigraphic units named from bottom to top: GI – GV. GI unit is 19 m thick of interior facies in the base of the Galería Complex, divided into two sub-unit, GIa and GIb, by the Matuyama-Bruhnes paleomagnetic boundary. GI unit shows an issue with the chronology since has uncoherent between TT-OSL and ESR/U-series and paleomagnetism dates. This work has been done by combining field observation with laboratory sedimentary analysis to characterize the texture and structure of the sediments. Based on these studies, 12 layers and 9 sedimentary facies have been identified. The facies associations indicate a clear separation between GIa and GIb sub-units. GIa sub-unit is dominated by epiphreatic conditions and represents continuous relativity sedimentation during the Early Pleistocene; meanwhile, GIb shows important erosion events and facies with reworked materials that indicate vadose conditions during the Middle Pleistocene. This environmental change is related to the geomorphological evolution of the Arlanzón River. In addition, soft-sediment deformation structures have been described, including faults and low-angle folds. An important leaching process has been identified by the presence of phosphates that could explain the underestimated ages obtained in other works.

Revision of TD1 and TD2 stratigraphic sequence of Gran Dolina cave (Sierra de Atapuerca, Spain)

Gran Dolina is a cavity infilled by at least 25 m of Pleistocene sediments divided into 12 lithostratigraphic units and 19 sedimentary facies. These sedimentary facies have been divided into allochthonous facies, defined as sediment inputs from the outside, and autochthonous facies, defined as sediments generated within the karst; but this division has been challenged in recent works. In this study, TD1 and TD2 units of Gran Dolina have been detailed studied and the use of autochthonous facies has been assessed. For that purpose, we have studied the stratigraphic excavation profile, combining field observation with laboratory sedimentary analysis (sieving, laser diffraction, and XRD) to characterize the texture and structure of the sediments. Based on these studies, a total of 8 sedimentary facies have been identified. Consequently, TD1 unit has been separated into two sub-units and 13 layers, while the TD2 unit has been divided into three sub-units. The facies associations indicate a succession of phreatic and vadose phases that would define together epiphreatic conditions inside the cave, related to the transition between Arlanzón valley terraces T3 and T4. Interior facies (and entrance facies for allochthonous facies) is proposed to define Gran Dolina's sediments since the facies analyses indicate transport by underground flows.

Seismic facies, stratigraphy, geomorphology, and seismic modelling of a Lower Cretaceous carbonate platform

Abstract Globally, Lower Cretaceous carbonate platforms form important hydrocarbon reservoirs. An exceptional seismic dataset allowed us to examine controls on the stratigraphy, seismic geomorphology, facies architecture and along-strike variability of a Lower Cretaceous platform. Within the platform succession, a partitioning of facies is developed between Transgressive, Highstand and Forced Regressive seismic sequences. Spectacular seismic geomorphological images of the platform using spectral decomposition techniques reveal spatial organization within both depositional and diagenetic (i.e. palaeokarst) patterns. The study was undertaken in order to optimize a well location. Predrill seismic facies interpretations were interpreted with the aid of offset well data and drill cores. Core data encompass a range of slope, platform margin and platform interior facies with inferred palaeoenvironments that are matched to the seismic geomorphology of the platform. 2D seismic forward modelling was integrated in our workflow to investigate the extent of potential sealing lithologies and the reservoir potential of the individual seismic sequences. Nearby wells were used to calibrate a modified rock physics model for the different lithofacies. Multiple seismic model realizations were generated to assess reservoir quality in alternative well locations and the continuity and quality of sealing strata. Results of the predrill forward-modelled poro-perm prediction are presented, showing a good fit with the final well observations after drilling.

Analysis of the relationship between porosity and permeability in reservoir modeling using the petrophysical rock type approach

The carbonate reservoir is one of the reservoir characters found in hydrocarbon fields in Indonesia. Carbonate reservoirs have complex porosity and permeability relationships. So it is necessary to do a special reservoir character that is different from the siliciclastic reservoir. Efforts that can be made to assist the development of this hydrocarbon field are to analyze the reservoir character in more detail using the petrophysical rock type (PRT) approach. This approach is used by combining geological elements such as the depositional environment, the petrophysical properties of the rock, as well as the fluid flow in it which is reflected by capillary pressure and water saturation. Modeling with this method is expected to be a method that can increase hydrocarbon production optimally in Xena Field. The object of research from Xena Field is Zone A2 which is included in the Parigi Formation. The Parigi Formation is one of the main hydrocarbon-producing reservoirs. The data used in this study are routine core analysis (RCAL) rock data on JLB-07, JLB-08, JLB-02, JLB-23 wells, wire log data (gamma-ray log, resistivity log, density log, neutron log) of 30 wells, and 2D seismic data. The depositional facies are divided into 2 facies, namely the margin reef platform facies and the interior platform facies. Identification of rock type (RT) using the flow zone indicator (FZI) method. The rock type in this field can be divided into 4 rock types, namely RT 1, RT 2, RT 3, RT 4 with RT 1 being able to drain the best fluid and RT 4 to drain the worst fluid. Reservoir property modeling is controlled by facies and rock type (RT) models. The margin reef platform facies are associated with RT 1 and RT 2. The interior platform facies are associated with RT 2 and RT 3.

Facies and sequence stratigraphic architecture of the Mural Limestone (Albian), Arizona: Carbonate response to global and local factors and implications for reservoir characterization

AbstractThis study documents the detailed facies and sequence stratigraphic architecture of a multi‐cyclic patch‐reef and its associated ramp interior facies that formed during Oceanic Anoxic Event 1b in the Mural Limestone, Arizona, USA. Ramp interior facies are comprised of bedded wackestone/packstone, rudist build‐up and coral–algal patch‐reef facies located north of Bisbee, Arizona, at the Grassy Hill locality. The larger multi‐cyclic patch‐reef that developed coevallyca5 km to the south of Grassy Hill consists of a high‐angle windward margin with a narrowca70 m long reef frame containing vertically zonatedMicrosolena, Actinastrea, diverse branching coral and rudist assemblages, and an 870 m long low‐angle leeward margin comprised of reef debris rudstone and grainstone shoal facies. Similar reef geomorphology and orientation is documented across the Gulf of Mexico and reflects the shelf‐wide north to north‐east‐trending prevailing wind and current energies. Controls affecting reef formation and growth patterns include changes in accommodation space associated with low‐amplitude global sea‐level rise and regional thermotectonic subsidence, local accommodation space and nutrient fluctuations associated with the inner shelf depositional setting within a humid and siliciclastic‐rich environment. Four aggradational to retrogradational high‐frequency sequences are documented in Arizona: High‐frequency sequences 1 and 2 represent the first pulse of patch‐reef development in an overall second‐order marine transgression over the Sonora/Bisbee Shelf. These sequences correlate to δ13C signatures associated with Oceanic Anoxic Event 1b across the Gulf of Mexico and suggest that carbonate reefs persisted on the ramp interior during this time. High‐frequency sequences 3 and 4 record a second brief transgression and backstepping of reef facies followed by the final regression of shallow shelf carbonates that correlates to more robust patch‐reef development in Sonora, Mexico. The patch‐reef at Paul Spur is an excellent outcrop analogue for productive patch‐reefs in the Maverick Basin (Comanche Shelf) of Texas. Detailed facies mapping of this outcrop analogue shows that the greatest reservoir potential is contained within the backreef grainstone shoals where primary porosity of up to 15% is observed.

Petrogenesis of leucosome sheets in migmatitic UHP eclogites—Evolution from silicate-rich supercritical fluid to hydrous melt

Recent studies of eclogite in the ultrahigh pressure (UHP) zone of the central Sulu belt in China have shown that phengite remained stable during exhumation from the metamorphic peak to HP eclogite facies conditions. This observation requires that some other source of fluid was involved in the production of melts generated during exhumation from UHP metamorphic conditions. Here, we investigate decimeter- to meter-thick leucosome sheets in retrogressed and migmatitic UHP eclogite in the central Sulu belt. The leucosomes have variable mineral modes and deformation fabrics; some are composite with marginal and interior facies, whereas others are not. They range from granite to trondhjemite, with high abundances of SiO2, K2O + Na2O and Al2O3, similar to hydrous melts, and have major oxide and trace element variations consistent with modal changes in phengite and albite + epidote sensu lato. The leucosomes are enriched in large ion lithophile elements relative to high field strength elements and have rare earth element (REE) patterns enriched in light REE relative to heavy REE, generally with small positive or negative Eu anomalies. Newly-crystallized zircon in the leucosomes records weighted mean ages of c. 223–218 Ma. Early-formed leucosome has Sr-Nd isotope compositions similar to those of nearby unretrogressed UHP eclogites, whereas younger leucosome has Sr-Nd isotope compositions intermediate between the eclogites and surrounding gneisses. Ti-in-zircon thermometry combined with Si-in-phengite barometry indicates crystallization of the leucosomes between ~850 and ~770 °C, over a wide range of pressure (P) from 3.5 to 2.2 GPa, which correlates with age from older (higher P) to younger (lower P). At the metamorphic peak, which may have exceeded 5.5 GPa, the source rocks were likely fluid deficient or fluid absent. During exhumation from UHP conditions, we posit that exsolution of water stored in nominally anhydrous minerals formed a silicate-rich supercritical fluid in eclogite that evolved to a denser, more viscous and more polymerized hydrous melt. Phengite barometry indicates that the early-formed leucosomes crystallized at pressures close to the critical line for the basalt plus water system, possibly by diffusive loss of water to the host eclogite. Infiltration of solute-rich supercritical fluid from the surrounding gneisses blended with melt in the eclogites generating variable Sr-Nd isotope composition intermediate between these end-members, as recorded by leucosomes that crystallized at lower pressures. The youngest leucosomes crystallized at pressures near the critical line for the Ca-granite plus water system creating a low volume of aqueous fluid that precipitated quartz veins associated with the leucosome sheets, particularly at their margins. Subsequently, limited phengite-breakdown melting in the leucosomes is recorded by aggregates of plagioclase + biotite around phengite and thin films and cuspate veinlets and patches of K-feldspar along grain boundaries. Phase equilibrium modelling indicates that this late stage melting occurred at P–T conditions near the transition from HP eclogite to amphibolite facies, with final subsolidus equilibration at 1.0–0.9 GPa and T < 640 °C.

Chronology of the cave interior sediments at Gran Dolina archaeological site, Atapuerca (Spain)

The so-called “Gran Dolina site” (Atapuerca mountain range, N Spain) is a karstic cavity filled by sediments during the Pleistocene, some of which contain a rich ensemble of archaeological and paleontological records. These sediments have contributed significantly to our understanding of early human dispersal in Europe but, in contrast, older, interior facies deposits have received much less of attention. The stratigraphy of Gran Dolina reveals an abrupt sedimentary change of interior to entrance facies from bottom to top, reflecting a significant paleoenvironmental change that promoted the accumulation of sediments transported from the vicinity of the cave by water or “en masse”. Since the major magnetic polarity reversal known as the Matuyama-Brunhes boundary (0.78 Ma) was detected within the TD7 unit in the middle of the stratigraphic section, we carried out a new combined paleomagnetic, radiometric (U-Pb), and electron spin resonance (ESR) dating study of the lower part of the sequence in order to constrain the chronology of the interior facies at Gran Dolina. U-Pb analysis of speleothems did not produce age information as the samples proved to be extremely unradiogenic. The magnetic stratigraphy of the cave interior sediments reveals a dominant reverse magnetic polarity, coherent with a Matuyama age, and interrupted by a normal polarity magnetozone interpreted as the Jaramillo Subchron (1.0–1.1 Ma). ESR ages on quartz grains from the upper part of the interior facies sediments are coherent with such an interpretation. We conclude that the fluvial deposits (interior facies) that constitute the cave floor began accumulating before 1.2 Ma. The development of large cave entrances at Gran Dolina occurred shortly after the Jaramillo Subchron but before ca 900 ka ago.

Morphology and deformational history of Tellus Regio, Venus: Evidence for assembly and collision

Tessera terrain is the oldest stratigraphic unit on Venus, but its origin and evolution are inadequately understood. Here we have performed detailed mapping of Tellus Regio, the third largest tessera plateau on Venus. Tellus Regio is shown to have distinct marginal and interior facies. The east and west margins of Tellus rise up to 2 km above the interior and include ridges and troughs ∼5–20 km across, oriented parallel to the present plains-tessera boundary. Structures characteristic of the interior of Tellus are found within the eastern and western margins and are deformed by the margin-parallel ridges indicating their presence during the time of the formation of the current margins. These relationships suggest that the margins formed by the application of external horizontal compressional stresses at the edges of an already-existing tessera interior. Structural and stratigraphic relationships in southwest Tellus show the assembly of three structurally distinct tessera regions and intervening plains that are consistent with the collision of the southwest margin into the plateau interior. This requires that tessera terrain was formed regionally and collected into the present day Tellus plateau. The latest stages of activity in Tellus include volcanism and pervasive, distributed, 1–2 km wide graben, which may have been formed due to large-scale gravitational relaxation of the plateau topography. A large intratessera plains unit may have formed via crustal delamination. The collisional oroclinal deformation of the margins are most consistent with models that invoke mantle downwelling for the origin of Tellus Regio and other tessera plateaus with similar structural relationships.

Stratigraphic Hierarchy and Shelf-to-Basin Architecture of Aptian–Albian Mural Shelf, Cerro Caloso Range, Sonora, Mexico

The Mural-Caloso Shelf in Sonora, Mexico is one of only a limited number of outcrops that provide continuous shelf-to-basin profiles of reef-rimmed carbonate platforms with well-defined lowstand deposits and documentable downward shifts in coastal onlap. This paper examines the shelf-to-basin transition across three thirdorder sequences at the Cerro Caloso outcrop. Four parameters: reef framestone lateral and vertical dimensions, fore-reef slope lateral and vertical dimensions, evolution of platform interior facies, and presence or absence of lowstand systems tract siliciclastic wedges with or without mixed oolitic grainstone were used as the basis for the characterization of the shelf to basin profile evolution. The lower two sequences (S3 and S4) are mostly aggradational with broad coral-rudist reef rimmed margins and thick platform interiors dominated by carbonate facies. The uppermost sequence (S5) is progradational and downstepping with a narrow coralrudist reef rimmed margin and a thin siliciclastic-rich platform interior. A lowstand wedge of fine-medium grained sandstone and cross-bedded oolitic grainstones that onlaps the S4 coralrudist margin documents downstepping and a minimum of 20 m of downward shift in coastal onlap. The lower sequence wedge is relatively thin compared to the thickness of the S5 wedge. The long-term progradation and increased clastic component observed in the latter stages of shelf evolution indicate a large second order relative sea level fall during the deposition of the Lower Albian age Mural-Caloso Shelf. The continuous outcrop provides a good reference model for facies patterns and stratigraphic architecture for the time equivalent Cretaceous platforms that develop similar geometries such as the Shuiba shelf in the Middle East.

Record of the end-Permian extinction and Triassic biotic recovery in the Chongzuo-Pingguo platform, southern Nanpanjiang basin, Guangxi, south China

The Chongzuo-Pingguo platform is a vast isolated platform, 180 km across, in the southern part of the Nanpanjiang basin. The end-Permian extinction is recorded in conformable sections in the northern part of the platform (Pingguo area). Upper Permian skeletal packstone contains diverse open-marine fossils including Nankinella and Sphaerulina. It is overlain by a 4.6 m thick horizon of calcimicrobial framestone constructed by globular calcified microbial framework similar to Renalcis. The PTB event horizon is interpreted to occur at the top of the packstone, coincident with the abrupt change to calcimicrobial framestone lacking Permian macrofossils. The transition from Hindeodus latidentatus to H. parvus occurs 1 m above the base of the calcimicrobial framestone, marking the conformable biostratigraphic boundary. During the Early Triassic the platform developed as a low-relief bank rimmed with oolite shoals but was bordered by a high-relief, fault-controlled escarpment along its southern margin. Platform interior facies of the Majiaoling and Beisi formations are 900 m thick and consist of thin-bedded lime mudstone, oolite, and dolostone with restricted marine biota in four shallowing-upward packages that define depositional sequences. In the Chongzuo area the platform was terminated at the end of the Early Triassic by burial with up to 1600 m of felsic pyroclastic volcanics and lava flows. Transgression and shallow-marine carbonate sedimentation resumed early in the Middle Triassic followed by drowning in the Bithynian as indicated by a shift to deep-marine carbonate and clastic deposition. In the Pingguo area volcanic deposits are much thinner and only briefly interrupted carbonate accumulation. Here the platform interior drowned during a major back step early in the Middle Triassic (Anisian, Bithynian). Smaller pinnacle platforms of the Guohua Fm., up to 1700 m thick, continued to accumulate above the Early Triassic platform margin and were eventually drowned by the Late Pelsonian. In the Anisian, the pinnacle platforms shifted to sharply defined, reef-rimmed margins with Tubiphytes reefs. The shift coincides with accelerated metazoan biotic recovery in the Anisian. However, the reefs lack metazoan frameworks and are composed almost entirely of Tubiphytes reinforced by large volumes of marine cement. These observations indicate that controls beyond the recovery of framework-building metazoans governed the re-establishment platform-margin reefs in the Middle Triassic.

The Potsdam–Beekmantown Group boundary, Nepean Formation type section (Ottawa, Ontario): a cryptic sequence boundary, not a conformable transition

There are two unreconciled interpretations for the age and character of the boundary separating the Cambrian–Ordovician Potsdam and Beekmantown groups that underlie the Ottawa Embayment in eastern Ontario. These stratal groups consist of interior facies of the central Laurentian Platform. As exposed in the type section of the Nepean Formation (upper Potsdam Group), located in the City of Ottawa, the boundary was previously interpreted to be conformable and of Early Ordovician age. This intepretation was of enormous impact on subsequent regional geology compilations that showed a diachronous boundary across the platform interior. From recent subsurface analysis across eastern Ontario, the contact was interpreted to be disconformable, a sequence boundary separating Late Cambrian and Early Ordovician strata. This paper reexamines the type section. Lithologically, the group boundary should be repositioned downsection by ~1.5 m. The contact now lies coincident with a disconformity that has a paleorelief of &lt; 10 cm. The proposed revision is geologically significant. Previous collections of Early Ordovician conodonts from the type section, used to define the age of what had been interpreted to be upper Potsdam strata, now fall entirely within the lower Beekmantown Group. Nepean (Potsdam) strata exposed in the type section remain undated. Regional correlation of the disconformity across the Laurentian platform suggests that Nepean strata at the type section are likely of Late Cambrian age. There now exists a regionally coherent separation of Cambrian and Ordovician sedimentation patterns in the Ottawa Embayment.

Microbial carbonates as indicators of environmental change and biotic crises in carbonate systems: examples from the Late Devonian, Alberta basin, Canada

Microbial precipitation of calcium carbonate has played a vital role in the development of carbonate platforms since their initiation in the Proterozoic. We report here the varied roles that microbial carbonates played in Late Devonian carbonate platforms in the Alberta basin, Canada. We recognize microbial carbonates as important contributors within the carbonate system during times of major environmental change including transgressive events in platform environments and the recovery interval following the Frasnian–Famennian mass extinction. Detailed sequence stratigraphic analysis of two isolated platforms in the Canadian Rockies was used to document their evolution from a regional ramp to isolated platforms with phases of progradation, aggradation and backstepping, and renewed progradation related to rates of second-order and third-order sea-level change and basin infill. The carbonate system was reorganized following annihilation of many carbonate-producing biota during the Frasnian–Famennian mass extinction. Microbial carbonates figure prominently in both Frasnian platform development and the Famennian recovery of the carbonate system following the Frasnian–Famennian mass extinction. During transgressive platform phases deeper-water facies with a crinoid, rugose coral, and microbe dominated biota abruptly overlie prograding stromatoporoid framestones. Microbial carbonates consist of abundant Girvinellid oncoids that nucleated around other fossil clasts. The temporary replacement of stromatoporoid communities during transgression implies a shift from oligotrophic to mesotrophic conditions and the microbes likely capitalized on the change in nutrient supply. Microbes played a significant role in highstand reef margin facies where Renalcis was a binder and cementer in stromatoporoid framestones and downslope rugose coral/stromatoporoid mounds. Microbially laminated carbonates also form stromatolitic mats in peritidal shallowing-upward cycles and repetitive stromatolitic intervals commonly indicate sea-level lowstands associated with the development of sequence boundaries. Stromatoporoid reefs and their associated reef interior facies indicate an oligotrophic ecosystem where microbial carbonates were relegated mainly to cryptic or stressed marine habitats. An end-Frasnian sea-level lowstand exposed the isolated carbonate platforms. Transgression and mass extinction characterize the Frasnian–Famennian boundary event and microbial carbonates occur at this horizon in western Canada. Stromatolites, oncoids, and large-scale microbial thrombolites appear to be opportunistic taxa temporarily filling unoccupied ecological niches immediately above the extinction boundary. The paucity of megafauna implies that few metazoans remained to exploit available food and nutrient sources and the stratigraphic position of these microbial carbonates supports their interpretation as ‘disaster forms’. Microbial carbonates thus play several significant roles in the development of Late Devonian carbonate systems in western Canada. Microbial carbonates occur at important sequence stratigraphic and paleoecologic horizons indicating changes in sea level, nutrient supply, and biotic assemblages. These examples indicate that microbial carbonates can be important indicators of environmental and ecological change within carbonate systems.

The Chemical and Isotopic Differentiation of an Epizonal Magma Body: Organ Needle Pluton, New Mexico

Major and trace element, and Nd and Sr isotopic compositions of INTRODUCTION whole rocks and mineral separates from the Oligocene, alkaline A fundamental problem of igneous petrogenesis is the Organ Needle pluton (ONP), southern New Mexico, constrain origin of the high-silica magmas that are tapped by largemodels for the differentiation of the magma body parental to this volume ash-flow tuffs (Hildreth, 1981). Many workers compositionally zoned and layered epizonal intrusive body. The agree that high-silica magmas are products of fractional data reveal that the pluton is rimmed by lower eNd (~ –5) and crystallization and crystal–liquid separation along the higher Sr/Sr (~0·7085) syenitic rocks than those in its interior side walls of large epizonal magma bodies, which together (eNd ~ –2, Sr/Sr ~0·7060) and that the bulk compositions create residual, differentiated magmas that buoyantly rise of the marginal rocks become more felsic with decreasing structural along the side walls and pool at the chamber roof (e.g. depth. At the deepest exposed levels of the pluton, the eNd ~ –5 Sparks et al., 1984; McBirney et al., 1985; Feeley & lithology is a compositionally heterogeneous inequigranular syenite. Davidson, 1994; Spera et al., 1995). However, except for Modal, compositional and isotopic data from separates of rare the studies of Sawka et al. (1990) and Mahood & Cornejo earth element (REE)-bearing major and accessory mineral phases (1992), little direct evidence of these processes has been (hornblende, titanite, apatite, zircon) demonstrate that this decoupling described from ash-flow tuffs or exposed epizonal plutons. of trace and major elements in the inequigranular syenite results The paucity of evidence from ash-flow tuffs reflects their from accumulation of light REE (LREE)-bearing minerals that selective and possibly spatially biased sampling of the were evidently separated from silicic magmas as the latter rose along entire parental magma body (Wilson & Hildreth, 1997). the sides of the magma chamber. Chemical and isotopic data for Also, although epizonal plutons may expose the crysmicrogranular mafic enclaves, as well as for restite xenoliths of tallized remnants of an original magma body, these Precambrian granite wall rock, indicate that the isotopic distinction commonly show evidence of subsolidus chemical rebetween the marginal and interior facies of the ONP probably equilibration (Hildreth, 1981). reflects assimilation of the wall rock by eNd ~ –2 mafic magmas A rare example of an epizonal intrusive rock that near the base of the magma system. Fractional crystallization and has preserved evidence of the processes involved in crystal–liquid separation of the crustally contaminated magma at the differentiation of shallow-level silicic magmas is the the base and along the margins of the chamber generated the highly reversely zoned Organ Needle pluton (ONP), one phase silicic magmas that ultimately pooled at the chamber top. of the Oligocene, alkalic Organ Mountain batholith of

Development of a carbonate platform with potential for large discoveries - an example from Vietnam: ABSTRACT

In offshore central and southern Vietnam a number of carbonate accumulations can be recognized. Platform carbonates form basin-wide units of carbonate characterized by strong, continuous parallel seismic reflectors. Facies are dominated by bioclastic wackestones with poor-moderate reservoir quality. On the more isolated highs, large buildups developed. These are typically 5-10 km across and 300 m thick. They unconformably overlie the platform carbonate facies which are extensively karstified. In places these are pinnacles, typically 2-5 km across, 300 m+ thick with chaotic or mounded internal seismic facies. The large carbonate buildups are characterized by steep sided slopes with talus cones, reef-margin rims usually developed around only part of the buildup, and a prominent back-stepping geometry. Buildup interior facies form the main potential reservoirs They are dominated by fine to coarse grained coralgal packstones. Fine grained carbonates are associated with deeper water events and multiple karst surfaces can also be identified. Reservoir quality is excellent, largely controlled by extensive dissolution and dolomitization believed to be related to the exposure events. Gas has been found in a number of reservoirs. Heterogeneities can be recognized which could potentially effect production. These include the extensive finer grained facies, cementation or open fissures associated with themore » karst surfaces, a more cemented reef rim, shallowing upwards facies cycles and faults.« less

Microfacies associations in Asbian carbonates: an example from the Urswick Limestone Formation of the southern Lake District, northern England

Abstract Semi-quantitative analysis of allochems from the Urswick Limestone Formation (Asbian) of the southern Lake District area of northern England has revealed a distinctive cyclicity of the microfacies. Cycle-top grainstone microfacies contain an algal flora comprising Koninckopora, Anatolipora and Polymorphocodium , with Girvanella filmaents and Ortonella lumps. Other allochems include intraclasts, large peloids and thick-shelled bivalves and gastropods. The middles of cycles are mostly packstones and micro-grainstones and contain allochems dominated by small peloids and the algae Kamaeena, Kamaenella and Epistacheoides , with the microproblematicum Ungdarella and relatively high abundances of micritic-walled formainifera such as endothyrids. Cycle bases contain a diverse algal assemblage including Coelosporella and Stacheoides , with other allochems represented by trilobites, ostracodes, Saccamminopsis , foraminifera such as Archaediscidae, the base late Asbian guide Howchinia , the base early Asbian guides Gigasbia gigas and Vissariotaxis , bivalves, small gastropods, bryozoans, sponge spicules and bored grains. Other allochems are found throughout most cycles, decreasing only in the very shallowest (intertidal) facies, or have an irregular distribution, and include brachiopod debris, crinoid ossicles and coral fragments. There are significant variations in allochem distribution according to palaeogeography. Close to the shelf margin there are higher abundances in the cycle top grainstones of the algae Koninckopora and Anatolipora , and also of the calcified filaments Girvanella and Ortonella , with thick-shelled gastropods, intraclasts and coarse peloids. At cycle bases, echinoderm arm plates and bryozoans are particularly abundant in packstone-wackestone textures. Platform interior facies are differentiated into a diverse open-marine type, with a high total abundance of bioclasts in the cycle base pack-wackestones including trilobites, Coelosporella, Stacheoides, Kamaena and bored grains, grainstones are dominated by small peloids, Kamaenella and Ungdarella ), and a more restricted cycle type, in which total bioclast abundances are low.

The carbonate factory of Middle Triassic buildups in the Dolomites, Italy: a quantitative analysis

ABSTRACTMiddle Triassic carbonate buildups of the Dolomites were high in relief (500–1000m) and small in size (one to a few square kilometres in area). A paradox results from the carbonate platform model that invokes the platform top, including reef rims, as the carbonate factory and flanking beds as talus deposits. Most buildups consist largely of clinoforms (inclined at 10‐50°) whereas massive reef rocks and stratified buildup interiors are poorly developed or absent. Facies and modal analysis of 323 thin sections from buildups of the Marmolada indicate that clinoforms are: (i) predominantly composed of in situ boundstones (56% of all samples); (ii) primarily made up of early cements (37 vol.%), microbial crusts (17 vol.%), micritic intraclasts (10 vol.%) and Tubiphytes (8 vol.%); and (iii) contain diagnostic shallow water grains (dasyclads, coated grains) that are less abundant by 1‐2 orders of magnitude compared with buildup interior facies. These data suggest that the clinoforms themselves were the main carbonate factory of the Triassic buildups. Stratified buildup interior rocks and massive reef rocks were apparently not a prerequisite for buildup growth and clinoform progradation.

Solvent Tracers And The Judy Creek Hydrocarbon Miscible Flood

Abstract Radioisotopic liquid hydrocarbons have been successfully used as solvent tracers for the Judy Creek Beaverhill Lake "'A" Pool horizontal hydrocarbon miscible flood. Tritiated toluene and tritiated normal alkanes from pentane to octane were selectively injected as tracers into miscible flood patterns during solvent injection cycles. Subsequent tracer production at wells surrounding the miscible injectors provided a means to confirm each pattern's contribution to tertiary oil response at those wells. Communication with pattern injectors was an important factor to consider when solvent sweep was estimated and candidate areas were evaluated for pattern reconfiguration. Tracing program results were used to identify solvent migration from pattern areas. Field engineers used the information on flow distributions to advantage when they made changes to injection and production rates in order to improve pattern balancing. This paper includes a description of a new analysis technique which uses fractional distillation and gas chromatography to determine tracer activities in produced oil samples. Details of tracer injection and sampling strategies are also discussed. Introduction The Judy Creek Beaverhill Lake (BHL) "A" Pool is located 240 Ian northwest of Edmonton. The pool is an atoll-like reefal limestone complex of upper Devonian age. The reef has an areal extent of approximately 115 km2 and is up to 70 m thick. Together, the reef and platform limestones comprise the Swan Hills Formation of me Beaverhill Lake Group. Figure 1 shows a structural cross section of the reef along a line drawn from the northeast to the southwest. It is divided into live growth cycles. The uppermost production interval is the R-5, which is generally continuous with 90% of its porosity greater than 6%. This interval contained 41%of the pool's original-oil-in-place (OOIP). The remaining four cyclescan be grouped into interior and margin areas based on facies, with 32% of the pool's OOIP in the highly continuous reef margin. The best reservoir is found in the forereef, to the northeast. In the stratified reef interior, porosity is discontinuous and occurs as patches and lenses(1). The pool was discovered in 1959. Water injection commenced in 1964 into down-dip pool edge locations to help maintain reservoirressure. It was found that the injected water followed the highly permeable reef rim; the relatively impermeable interior facies presented barriers to fluid movement to the reefs interior. The severe pressure gradients that developed provided convincing evidence of the reservoir's complexity. In 1974 a pattern waterflood was initiated and water injection into the peripheral injectors was reduced. These actions gradually corrected the pressure imbalance across the reservoir. Approval for a horizontal hydrocarbon miscible flood was received from Alberta's Energy Resources Conservation Board in 1983. In Mar 1985, solvent injection commenced into four patterns of what was then planned to ultimately be a 56-pattern commercial project. Figure 2 shows the miscible flood patterns in relationship o the project boundaries as of February 1990. Hydrocarbon miscible flood surveillance included an extensive solvent radioisotope tracing program. As described by Jonasson(2), the program's objectives were to provide information on continuity, directional now trends and solvent flow beyond miscible patternboundaries.

Wolfcampian Sequence Stratigraphy of the Eastern Central Basin Platform, Texas

Integrated study of well logs, cores, high-resolution seismic data, and biostratigraphy has established the sequence framework of the Atokan (Early Pennsylvanian)-Wolfcampian (Early Permian) stratigraphic section along the eastern margin of the Central Basin platform in the Permian basin. Sequence interpretation of high-resolution, high-fold seismic data through this stratigraphic interval has revealed a complex progradational/retrogradational evolution of the platform margin that has demonstrated overall progradation of at least 12 km during early-middle Wolfcampian. Sequence stratigraphic study of the Wolfcamp interval has revealed details of the internal architecture and morphologic evolution of the contemporaneous platform margin. Two generalized seismic facies assemblages are recognized in the Wolfcampian. Platform interior facies are characterized by high-amplitude, laterally continuous parallel reflections; platform margin facies consist of progradational sigmoidal to oblique clinoforms and are characterized by discontinuous, low-amplitude reflections. Sequence interpretation of carbonate platform-to-basin strata geometries helps in predicting subtle stratigraphic trapping relationships and potential reservoir facies distribution. Moreover, this interpretive method assists in describing complex reservoir heterogeneities that can contribute to significant reserve additions from within existing fields.

Geology and Reservoir Description of a Dolomitized Upper Devonian Reef, Westerose Field, Alberta: ABSTRACT

The Westerose field produces hydrocarbons from a dolomitized Upper Devonian Leduc reef. The reef is 1.6 km wide, 5.5 km long, and up to 175 m thick. Core description, statistical analysis of whole core data, well logs, and pressure buildup data, were integrated in the development of a reservoir model. In contrast to well-known limestone reservoirs in western Canada, the organic reef margin facies and reef interior facies are not strongly differentiated. The reef interior does not contain laterally extensive barriers to vertical fluid flow. The secondary pore system dominates porosity. The large solution vugs and fossil molds are well connected through the dense matrix by a vertical fracture system. Porosity and permeability increase from the reef flank to the interior and are optimum within the lower reef interior. The reservoir model, interpreted from pore system distribution and supported by reservoir pressure response, is a disordered arrangement of permeability blocks. Cumulative probability plots indicate that the horizontal permeability data set is top truncated, with up to 35% of the highest permeability values not represented in the data set. Examination of a Leduc equivalent outcrop yielded data on pore-size frequency and porosity at scales larger as well as smaller than coremore » diameter. The sample size necessary to make representative measurements of reservoir properties can be estimated from the outcrop analysis.« less

Depositional History of Upper Triassic Carbonate Platforms on Wrangellia Terrane, Western British Columbia, Canada: ABSTRACT

Upper Triassic (Karnian-Norian) limestones of the Quatsino Formation, Vancouver Island, and the Kunga Formation, Queen Charlotte Island, were deposited within a complex rifted island arc system, which was accreted to the western margin of North America as Wrangellia during the mid-Cretaceous. Carbonates sedimentation began after a transgressive event submerged the subaerially exposed Karmutsen volcanics. In both Vancouver and Queen Charlotte Islands, the lower part of the carbonate sequence records the widespread development of a shallow, subtidal platform composed mainly of two facies: open-platform deposits (bioturbated fossiliferous wackestones) and platform sand-shoal deposits (oolitic-bioclastic grainstones). This early depositional stage evolved gradually into a second stage characterized by the fragmentation of the platform and development of a carbonate bank/basin topography controlled by extensional tectonics within Wrangellia. The interior facies of these carbonate banks consist of cyclic subtidal lagoonal and tidal-flat limestones, each cycle commonly capped by a vadose diagenetic layer. The bank margin facies comprises exclusively high-energy, oolitic sandshoal deposits. The adjacent basin facies include (1) pelagic/hemipelagic limestones, (2) platform-derived and slope-derived carbonate breccia, and (3) proximal to distal carbonate turbidites. In both island systems, carbonate sedimentation ended abruptly when the isolated carbonate banks were drowned and covered by basinal clastics in responsemore » to rapid subsidence probably caused by tectonic activity within Wrangellia.« less