Platform Interior Facies Research Articles

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.

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.

Facies development of a Late Ordovician mixed carbonate-siliciclastic ramp proximal to the developing Taconic orogen: Lourdes Formation, Newfoundland, Canada

The Upper Ordovician (late Whiterockian to Mohawkian) Lourdes Formation represents a narrow (tens of kilometers), short-lived [∼5–7 million years (my)], open-ocean (high-energy) mixed siliciclastic-carbonate ramp that onlapped allochthonous strata along the orogen side of the local Taconic foreland basin. Platform development followed a 6–8 my hiatus during which weathering had concentrated chemically mature siliciclastics that were admixed with initial carbonate sediments. A cross-platform facies gradient contains paleokarst and peritidal carbonates and sandstones, shallow-ramp carbonate bioherms and skeletal shoals, and deeper ramp calcareous shales. Transgressive systems tracts are marked by ramp-wide sheets and shoals of skeletal grainstone and low accumulation rates, and highstand systems tracts are marked by significant admixture and interbedding of siliciclastics with cross-ramp carbonate facies. Platform demise coincides with increased siliciclastic input, which is likely tectonically influenced. The Lourdes platform is equivalent to epicontinental foreland ramps along eastern Laurentia, but its narrower width precluded formation of oceanographically restricted platform-interior facies.

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.

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.

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.

Sequence Stratigraphy and Systems Tract Development of the Latemar Platform, Middle Triassic of the Dolomites: Outcrop Calibration Keyed by Cycle Stacking Patterns

The Middle Triassic Latemar platform provides a seismic-scale outcrop example of an intact carbonate shelf-to-basin transition, ideal for integrating sequence stratigraphy with facies and cyclic stratigraphy. This subcircular, high-relief buildup records two third-order accommodation sequences within the platform interior: the lower Ladinian sequence and the upper Ladinian sequence. Sequence L1 developed atop a widespread, low-relief Middle Anisian carbonate bank (60 m thick). Underlying subtidal bank cycles thin upward into the basal, subaerial sequence boundary (type 1) reflecting decreasing third-order accommodation; above it, platform-interior facies of sequence L1 retrograde. This results in superimposition of Ladinian basinal and foreslope facies atop the underlying, horizontal, shallow-water bank along its periphery. The transgressive (TST) and highstand systems tract (HST) of sequence L1 (as well as L2) are marked by long-term, systematic vertical facies changes and variation in stacking patterns of aggradational high-frequency, 20 kyr cycles within the platform interior. The maximum flooding surface (MFS) is a marine hardground surface displaying evidence of very slow sedimentation and is the platform expression of the condensed section. A type 2 SB caps sequence L1, marked by an interval of vertically superimposed thin subaerial tepees; beneath this, high-frequency cycles are thinning-upward, and above they are thickening-upward. Only themore » transgressive systems tract of sequence L2 is preserved at the Latemar owing to late Ladinian-Early Carnian volcanism and tectonism which terminated carbonate platform deposition.« less

Facies and Geologic History of an Exposed Miocene Rift-Margin Carbonate Platform: Gulf of Suez, Egypt

Abu Shaar is a small exhumed carbonate platform of lower to middle Miocene age on a horst of Precambrian crystalline rock along the western side of the Gulf of Suez. The reef-rimmed complex is entirely dolomite but with exquisitely preserved primary and diagenetic microfabrics. Similar platforms, some of which contain hydrocarbons, are buried beneath evaporites in the Gulf. The platform developed in three stages, defined as local members of the Rudeis Formation. The first stage records deposition during progressive marine onlap as aprons of mixed terrigenous-carbonate sediments evolved into a wide reef-rimmed platform. Synsedimentary platform-margin collapse subsequently removed the most seaward portions of the reefs along the eastern Gulf-facing margin. These truncation surfaces were then covered by a second phase of reef growth and deposition of fore-reef carbonates. The third and final stage, deposited following a sea level drop and subaerial exposure, is a series of well-bedded peritidal carbonates that become progressively more evaporitic upward. Fore-reef facies are again truncated by synsedimentary slope failure. Former evaporites are now recorded as collapse breccias, contorted bedding, black chert nodules, and evaporite molds. Porosity is mainly secondary as vugs and molds and is highest in platform interior facies. These more porous sediments contained more aragonitic components originally and were less affected by submarine cementation. In contrast, the higher degree of synsedimentary lithification of platform-margin reef and fore-reef sediments is comparable with what is commonly observed in modern reef-dominated platform margins.