Carbonate-siliciclastic Sequences Research Articles

The unconventional petroleum system of a mixed siliciclastic-carbonate sequence: Irati Formation, Paraná Basin, Brazil

The unconventional petroleum system of a mixed siliciclastic-carbonate sequence: Irati Formation, Paraná Basin, Brazil

Calcareous nannofossils, planktonic foraminifera, and revised stratigraphy of the Eocene Çayraz Formation; the final stage of marine sedimentation in Central Anatolia, Turkey

New field observations and discovery of calcareous nannofossils and planktonic foraminifera from the 'shallow-marine' Çayraz Formation (Haymana Basin, Central Anatolia), a contributing Eocene section to the shallow benthic zonation (SBZ) in the Tethys, allow us to revise its stratigraphy and establish an integrated biostratigraphic scheme for the first time. The hemipelagic marls in the uppermost part of the Eskipolatlı Formation that underlies the Çayraz Formation yielded nannofossil assemblages of Zone CNE3, pinning down the initiation of the Çayraz shelf system into the 'middle' Ypresian. The prominent marly part (Member B) between the carbonate-clastic packages of the Çayraz Formation with prolific occurrences of larger benthic foraminifers (LBFs) (Members A below and C above) yielded calcareous nannofossils suggesting Zone CNE6 (late Ypresian). We show that the mixed carbonate-siliciclastic sequence with abundant LBFs in the upper part of the formation (Member C) is overlain by newly discovered hemipelagic marls (here named as Member D). These marls yielded calcareous nannofossils indicating Zone CNE9 and CNE10 for the lower and Zone CNE12(?Lutetian) for the upper samples. The same beds yielded planktonic foraminifers indicating Zone E8 for the lower and Zone E9 for the upper samples. We conclude that shallow-marine sedimentation in the Çayraz section ended in the 'middle' Lutetian, challenging the previous Bartonian records by LBFs. A new lithostratigraphic scheme consisting of four members with distinctive lithological and biotic characteristics is proposed for the Çayraz Formation: two main shelf systems (Members A and C), each followed by deep-marine sedimentation (Members B and D).

Burial stylolites enhance karst dissolution and control cave geometry in heterolithic carbonates, Salitre Formation, Brazil

Understanding the karstification process in fractured carbonates is a key factor in developing and exploiting fractured and karstified reservoirs. This study documents the relationship among the 2D geometry, stratigraphy and petrophysical properties of a cave system developed in a Neoproterozoic mixed carbonate-siliciclastic sequence. We applied a multidisciplinary and multiscale approach that combined structural geological surveys, cave imaging by Laser Detecting and Ranging (LiDAR), linear scanlines, stratigraphic logs, uniaxial compressive strength (UCS), mercury-intrusion porosimetry, and 2D image analysis. We found that bedding-parallel stylolites in dolomitic limestone consist of mm-to cm-thick high-porosity zones (up to 20% measured via 2D image analysis). They acted as fluid conduits that enhanced bedding-parallel dissolution. Field and thin section analyses showed that beds with close spacing and thicker stylolites are more karstified and dissolved than beds showing greater spacing or completely devoid of stylolites, indicating that stylolitic zones act as flow pathways in low porosity-permeability carbonates. Primary porosity, pore size distribution, density and capillary pressure of host rocks have no direct relationship with karstification intensity. We conclude that clusters of burial stylolites may control the hypogenic cave geometry. These findings have direct implications for subsurface permeability prediction in karstified carbonate reservoirs.

Siliciclastic content controls the sealing behavior of layers and karst development in a carbonate-siliciclastic sequence, Salitre Formation, Brazil

Although caves formed in carbonate units play a crucial role in fluid flow, most subsurface voids are below seismic resolution. This study investigates how sedimentary facies control the vertical distribution and the development of karst features from small vugs to caves in a carbonate succession with terrigenous input. We analyzed the sedimentary succession and the potential of layers to behave as sealing and conduit units in the karst system. The study area is located in the Neoproterozoic Salitre Formation in the Una-Utinga Basin, São Francisco Craton, Brazil. We selected two caves and their hosting units as analogs of carbonate reservoirs affected by karstification. Our findings indicate that the caves have similar sedimentary and petrographic patterns, composed of three sedimentary facies at a 20–30 m vertical section from the bottom to the top. At the bottom, the lime mudstone facies contains 10% siliciclastic content, which is the pure conduit facies that host most cave passages. A heterogeneous calcarenite containing lime mudstones and siliciclastic intercalation facies occurs in the middle portion, where the siliciclastic content varies between 10-30% and 30–50%. This aforementioned facies exhibits conduit and sealing layers. At the top, a mixed limestone siliciclastic facies has siliciclastic content above 50% and a high degree of compaction. It behaves as a sealing facies. The conduit-sealing system has the same pattern at various scales: macro (observed in drone imagery), meso (Light Detection and Ranging imagery - LiDAR), and microscale (thin section). We conclude that the siliciclastic content of layers controls the degree of karstification at various scales in a carbonate-siliciclastic sequence.

Sequence Stratigraphy of Miocene Mixed Carbonate-Siliciclastic in Fold and Thrust Belt System, Banggai Basin

This study was specifically analyzed in the southern part of the Banggai Basin, precisely in the TKA Field. In the Tertiary, regional stratigraphy reveals, the Banggai Sula microplate comprises carbonate shelf and siliciclastic sequence. Several previous studies had been concluding, the tertiary limestone was deposited during the Early Eocene to Middle Miocene. Carbonate formations which has been known by Tomori and Minahaki Formation. Meanwhile, their siliciclastic dominated is Matindok Formation and Celebes Mollase. These sequences formed a distinct stratigraphic style in the geophysical well log and seismic data. Four cycles are distinguished, the lower part of Tomori Formation at the outer shelf and directly overlying by the upper part of inner shelf Tomori Formation. it was superimposed by siliciclastic sediments that have been deposited in the outer litoral during regressive phase. Whereas during rising sea level or highstand, the litoral system migrated to basinward wherein Minahaki Formation was deposited. Sequence stratigraphy provides a reliable methodology for correlate both carbonate and siliciclastic successions. It was analyzed based on 4 wells, 2D, and 3D seismic data which has been available. Sequence stratigraphy analysis using T-R sequence concept by Embry and Johanessen. Trasngressive and regressive phases were obviously determined by lithofacies, coral colony and foram that were recorded on biostratigraphic analyses. In this study, cyclic sea-level change will be recognized as system tract record of mixed carbonate-siliciclastic sequence.

Reservoir heterogeneity of an Eocene mixed siliciclastic-carbonate succession, northern Pannonian Basin

The quality of any reservoir is strongly controlled by diagenetic processes, especially of those that are comprised of mixed siliciclastic-carbonate sequences. An Eocene mixed siliciclastic-carbonate sequence forms reservoirs with diverse reservoir quality in the Hungarian Palaeogene Basin, which is found within the Pannonian Basin. Late Eocene transgression established extensive shallow-marine depositional environments strongly controlling the early diagenetic processes. Drill cores and cuttings were selected from several boreholes, representing a wide range of lithofacies to reveal the cause for reservoir heterogeneity. Petrographic, stable isotope and fluid inclusion analyses were performed to identify the diagenetic processes that controlled the porosity evolution of the reservoir, The early diagenetic calcite neomorphism caused lateral homogeneity, whereas the early meteoric water incursion enhanced reservoir properties via the development of dissolution pores, which in turn resulted in vertical reservoir heterogeneity. The deterioration of porosity started in the eogenetic realm by kaolinite formation and continued during mesodiagenesis as burial calcite, quartz, dolomite and minor illite cement precipitated in remaining pore spaces. Petrographic evidence, such as kaolinite transformation into dickite and fluid inclusion homogenisation temperatures highlight the role of temperature increase with burial, the effect of changing fluid chemistry during the evolution of the basin is documented by the chemical zonation of dolomite, the variation in salinities of the fluids enclosed fluid inclusions, as well as recurrent and prolonged periods of cementation with calcite, quartz and dolomite. Mesogenetic processes were accompanied by hydrocarbon emplacement and the formation of late-stage pyrite. The propagation and cementation of faults first contributed to increase and later to reduce fluid flow, respectively. These steps collectively resulted in a heterogeneous reservoir, characteristic of the Eocene sequence in the Hungarian Palaeogene Basin.

Silicification, flow pathways, and deep-seated hypogene dissolution controlled by structural and stratigraphic variability in a carbonate-siliciclastic sequence (Brazil)

Fractured and karstified carbonate units are key exploration targets for the hydrocarbon industry as they represent important reservoirs. Furthermore, large water reserves and geothermal systems are hosted in carbonate aquifers. This paper documents the relationships between stratigraphy, structural patterns, silicification, and the spatial-morphological organization of a 3D multistorey cave system developed in a Neoproterozoic mixed carbonate-siliciclastic sequence. We found that the combination of lithology, silicification, fracture patterns (controlled by lithostratigraphic variability), and petrophysical properties control the formation of high or low permeability zones; their distribution was fundamental for the spatial organization of dissolution and the compartmentalization of the resulting conduit system in different speleogenetic storeys. We propose a deep-seated hydrothermal origin for the fluids involved in the main phases of karst formation. Warm and alkaline hydrothermal fluids caused silica dissolution, followed by chalcedony and quartz reprecipitation in pore space and fractures. Rising fluids concentrated along through-going vertical fracture zones in the lower storey, whereas sub-horizontal bedding-parallel fluid flow was focused on sedimentary packages containing highly silicified dolostones (SiO2>80 wt%) characterized by high permeability. The Calixto Cave is an enlightening example for the complex speleogenetic history affecting a mixed carbonate-siliciclastic succession where the combined effect of silicification and hydrothermal karst dissolution can potentially generate high-quality reservoirs.

Factors controlling tight oil and gas reservoir development in the Jurassic siliciclastic-carbonate rocks in Sichuan Basin, China

The prediction of “sweet spot” of multi-lithology composite tight oil and gas reservoirs within mixed siliciclastic-carbonate sequences is a hot topic in oil and gas exploration. There are mixed lacustrine carbonate-siliciclastic rocks in the Da'anzhai Member of Ziliujing Formation in the Eastern Slope of western Sichuan Depression. The reservoir lithology is complex, and the main factors controlling the development of high-quality reservoirs are yet to be known. Based on a large number of drilling, core and thin section observation, well log and seismic data, this paper systematically studies the characteristics and factors controlling the development of the mixed siliciclastic-carbonate reservoir. The results show that the reservoir is composed primarily of coquina, sandstone, breccia and shale in lithology, and its development is mainly controlled by sedimentary microfacies, diagenesis, tectonism (fracturing) and paleogeomorphology during the sedimentary period. The effective fractures in reservoirs of the low-energy shell shoal facies are well developed with relatively good physical properties. With the change of sedimentary microfacies from low-energy shoal to high-energy shoal to arenaceous shoal, the hydrocarbon production capacity worsens step by step. Diagenesis, such as dissolution and fracturing, has a certain effect on reservoir physical properties. The palaeogeomorphic highs and slopes had well-developed fractures during the sedimentary period which are conducive to constructive dissolution, and thus they are favorable zones for reservoir development. The development of structural fractures further amplifies the influence of dissolution; thus the fracture zones are also favorable for the development of high-quality reservoirs. The favorable sedimentary facies zone and fracture development degree serve as the core factors for the formation of high-quality mixed reservoirs. The muddy limestone with certain dissolved pores and structural fractures deposited under the low-energy shell shoal setting is of relatively high-quality reservoir in the Da'anzhai Member.

Mixed carbonate-siliciclastic sequence development influenced by astronomical forcing on a distal foreland, Miocene Dam Formation, eastern Saudi Arabia

Given increasing importance of tertiary reservoirs in the Middle East, the Miocene (Upper Aquitanian to mid-Langhian) Dam Formation in eastern Saudi Arabia was cored along a 40 km transect, to develop a high-resolution sequence framework, and examine the role played by astronomical forcing, moderate glacio-eustasy (20- to 40-m sea-level changes), and tectonic deformation in this distal foreland setting. The Dam Formation (up to 80 m thick) contains updip siliciclastics which interfinger downdip with marine carbonates. Siliciclastic facies include paleosol-capped, red mudrock (mudflat), green and gray mudrock (near-shore siliciclastic lagoon), rare mud clast–bearing sandstone (fluvial), and massive very fine to medium sandstone (terrestrial sand sheets, rare tidal channels). Carbonate facies include brecciated carbonates (paleosols), lime/clay-clast quartz wackestone (lag gravel), carbonate laminites and microbial heads and mounds (tidal flat and shallow subtidal), argillaceous quartzose marl (near-shore lagoon), peloid mudstone-wackestone (restricted lagoon), variably quartzose skeletal peloid mudstone to packstone (open lagoon), oolitic grainstone (hypersaline beach, tidal channel), mollusk packstone (hypersaline lagoon border), and foram-mollusk packstone-grainstone (seagrass meadows). The Dam Formation contains eight sequences, each of which contains 2 to 4 parasequence sets or individual parasequences. Sequences and individual parasequences are capped by paleosols and/or erosion surfaces, and microbial laminite caps are rare. Bases of parasequences locally are veneered with ravinement gravel lags or siliciclastics beneath shallowing-upward subtidal carbonates, and lack intercalated deeper water facies. Statistical analysis of core-gamma ray (API) vs depth (m) shows that accumulation rates were ~ 3 to 4 cm/kyr, considerably faster than long-term accommodation rates (~ 1.2 cm/kyr), indicating deposition occurred for only 40% of the time, the remainder of the time being non-depositional during many short-term lowered sea levels, evidenced by the coeval oceanic δ18O curve. Spectral analysis of the core-gamma ray logs show astronomical forcing within the eccentricity, obliquity, and precessional bands. Spectral analysis indicates that the eight Dam sequences (average 10 m thick, range < 5 to 18 m) are a mix of long-term obliquity (~ 1.2 Myr) and long-term eccentricity (400 k.y.) cycles. The 3- to 4.4-m sets/parasequences are short term (~ 100 kyr) cycles, and the 0.7- to 1.25-m parasequences are obliquity and precessional cycles. Given the updip position of the study area, repeated glacio-estutatic sea-level changes caused repeated shallow flooding (< 20 m) and emergence, which generated paleosol-bounded or erosionally capped sequences and parasequences. Although the distal foreland was undergoing synsedimentary deformation and slow but locally variable subsidence, which influenced variable thickness of units, astronomically driven glacio-eustasy was the dominant influence on timing of sequence and parasequence development.

Reassessment of the age and depositional environments of the Eocene Çayraz Formation, a reference unit for Tethyan larger benthic foraminifera (Haymana Basin, central Turkey)

The Çayraz Formation, a reference unit for Tethyan Ypresian-middle/late Lutetian larger benthic foraminifera (Haymana Basin, central Turkey), is a thick (>350 m) ‘shallow-marine’ mixed carbonate-siliciclastic sequence, divided into lower, middle and upper units. The lower and upper units predominantly consist of rhythmic intercalations of limestone and marly/silty beds with abundant larger benthic foraminifera (LBF), and the middle unit is composed of massive fine siliciclastic sediments, mostly barren in LBF and macrofossils. By studying three sections (ÇAYA, ÇAYB, and YEŞ), encompassing the lower unit, we show that the Çayraz Formation (ca. 100 m-thick) displays notable vertical and lateral variability in benthic fauna and facies along an SE-NW transect at its type locality near Çayraz village. The distribution of LBF and facies patterns indicate a general shallowing upward trend, with more open marine environments in the easterly section (ÇAYA) and restricted shallow marine to continental environments with low diversity fauna in the westerly section (YEŞ). A depositional setting ranging from outer to inner-ramp with continental influence only in the westerly section is proposed. Key orthophragminid and alveolinid taxa from the lower unit of the Çayraz Formation indicate shallow benthic zones (SBZ) 9/10 for its lower part, and SBZ 12 for its upper part, suggesting a late Ypresian age. The massive marly/silty section of the middle unit of the Çayraz Formation overlies the Alveolina-bearing limestones in all sections and yielded abundant planktonic foraminifera suggestive of P9 Zone. This implies a sharp environmental transition from platform to basin across the Ypresian-Lutetian boundary.

The lead-up to the Sturtian Snowball Earth: Neoproterozoic chemostratigraphy time-calibrated by the Tambien Group of Ethiopia

AbstractThe Tonian-Cryogenian Tambien Group of northern Ethiopia is a mixed carbonate-siliciclastic sequence that culminates in glacial deposits associated with the first of the Cryogenian glaciations—the Sturtian “Snowball Earth.” Tambien Group deposition occurred atop arc volcanics and volcaniclastics of the Tsaliet Group. New U-Pb isotope dilution–thermal ionization mass spectrometry (ID-TIMS) dates demonstrate that the transition between the Tsaliet and Tambien Groups occurred at ca. 820 Ma in western exposures and ca. 795 Ma in eastern exposures, which is consistent with west to east arc migration and deposition in an evolving back-arc basin. The presence of intercalated tuffs suitable for high-precision geochronology within the Tambien Group enable temporal constraints on stratigraphic data sets of the interval preceding, and leading into, the Sturtian glaciation. Recently discovered exposures of Sturtian glacial deposits and underlying Tambien Group strata in the Samre Fold-Thrust Belt present the opportunity to further utilize this unique association of tuffs and carbonate lithofacies. U-Pb ID-TIMS ages from zircons indicate that Tambien Group carbonates were deposited from ca. 820 Ma until 0–2 m.y. before the onset of the Sturtian glaciation, making the group host to a relatively complete carbonate stratigraphy leading into this glaciation. New δ13C and 87Sr/86Sr data and U-Pb ID-TIMS ages from the Tambien Group are used in conjunction with previously published isotopic and geochronologic data to construct newly time-calibrated composite Tonian carbon and strontium isotope curves. Tambien Group δ13C data and U-Pb ID-TIMS ages reveal that a pre-Sturtian sharp negative δ13C excursion (referred to as the Islay anomaly in the literature) precedes the Sturtian glaciation by ∼18 m.y., is synchronous in at least two separate basins, and is followed by a prolonged interval of positive δ13C values. The composite Tonian 87Sr/86Sr curve shows that, following an extended interval of low and relatively invariant values, inferred seawater 87Sr/86Sr rose ca. 880–770 Ma, then subsequently decreased leading up to the ca. 717 Ma initiation of the Sturtian glaciation. These data, when combined with a simple global weathering model and analyses of the timing and paleolatitude of large igneous province eruptions and arc accretion events, suggest that the 87Sr/86Sr increase was influenced by increased subaerial weathering of radiogenic lithologies as Rodinia rifted apart at low latitudes. The following 87Sr/86Sr decrease is consistent with enhanced subaerial weathering of arc lithologies accreting in the tropics over tens of millions of years, lowering pCO2 and contributing to the initiation of the Sturtian glaciation.

Reservoir characterization and hydrocarbon accumulation in late Cenozoic lacustrine mixed carbonate-siliciclastic fine-grained deposits of the northwestern Qaidam basin, NW China

Abstract Late Cenozoic lacustrine mixed carbonate-siliciclastic sequences in the northwestern Qaidam basin (NW China) are mainly composed of high porosity and low permeability fine-grained rocks but have big potentials of oil-gas commercial production. However, how these fine-grained rocks generate high-quality reservoirs and accumulate hydrocarbons, remains poorly known. This study focuses on shallow-buried Shang Youshashan Formation (Middle–Late Miocene, N22) and Shizigou Formation (Late Miocene–Pliocene, N23) reservoirs and hydrocarbons in Xiaoliangshan area of the northwestern Qaidam basin. We conduct mineralogy, petrography and porosity and permeability analysis for reservoir rocks and biomarker analysis for hydrocarbons and potential source rocks. The results indicate that the N22 high-quality reservoirs are dominated by fractured marlstones and lime mudstones and constitute structural traps, whereas the N23 hydrocarbon reservoirs are mainly composed of siliciclastic-dominantly rocks and form stratigraphic-lithologic traps. Similar biomarker ratios between crude oil samples and Paleogene organic matter-rich rocks reveal that hydrocarbons in the shallow system were likely derived from deep-buried source rocks, rather than late Cenozoic lacustrine fine-grained rocks themselves. In this case, large-scale vertical fractures created by kink band structures in the northwestern Qaidam basin are supposed to facilitate hydrocarbon migration and accumulation. Preliminary production data and exploration well oil-gas test results demonstrate highest yields and thus large hydrocarbon potentials in the N22 marlstone and lime mudstone reservoirs. Collectively, we suggest that relatively high-permeability, fractured and carbonate-rich reservoirs on structural highs are favorable targets for future oil-gas exploration in the northwestern Qaidam basin. This petroleum system provides a typical and significant example for hydrocarbon exploration and development in mixed carbonate-siliciclastic fine-grained rocks of nonmarine petroliferous basins.

The early Cretaceous Mansourabad shale-carbonate hosted Zn–Pb (-Ag) deposit, central Iran: An example of vent-proximal sub-seafloor replacement SEDEX mineralization

The Mansourabad stratiform and stratabound Zn-Pb (-Ag) deposit is hosted by an Early Cretaceous siliciclastic-carbonate sequence deposited in southern Yazd basin, Iran. Mineralization is mainly hosted in silty limestone and dolomite. Based on petrographic studies three sulfide ore facies can be distinguished: feeder zone, bedded and massive-replacement ore facies. Textures include replacement, breccia, massive, framboidal, laminated and vein-veinlet. Sulfide minerals are predominantly sphalerite, galena, and pyrite. Dolomite and quartz are the major non-sulfide gangue minerals. Based on microscopic studies, sulfide mineralization in the Mansourabad deposit emplaced during two stages: fine-grained sulfide bands (stage I) are intricately interlayered with organic-rich beds and thin turbidite beds. They exhibit classic sedimentary textures, such as lamina and bedding, indicative of a synsedimentary origin. Coarser-grained stage II base metal sulfides show breccia and vein-veinlet’s textures, and are considered to form by replacement during sub-seafloor fluid flow. Dolomitization and silicification are the major hydrothermal alteration styles in the Mansourabad deposit, and they occur in all host rocks, but only in the immediate vicinity of the syn-sedimentary normal fault that served as the main conduit for the mineralizing fluids.Calcite minerals from the feeder zone and massive-replacement ore facies have 115–250 °C trapping temperature with salinities ranging from 3.55 to 19.82 eq. wt.% NaCl eq. δ34S values of pyrite, sphalerite and galena range from −18.8 to +3.2‰. The highest δ34S values correspond to the feeder zone (+3.2 to +2‰), and the lowest to the bedded ore facies (−18.8 to −9.9‰) and the massive ore (+2.6 to − 4.7‰). The overall range of δ34S is remarkably higher than typical magmatic values, suggesting that sulfides formed from the reduction of seawater sulfate by bacteriogenic sulfate reduction in a closed or semi-closed system in the bedded ore, whereas thermochemical sulfate reduction likely played an important role in the feeder zone. The δ13CPDB and δ18OSMOW values of host limestone’s and hydrothermal dolomites range from +2.7‰ to +2.0‰, +8.6‰ to +9.2‰, +1.5‰ to +2.8‰ and +10.6‰ to +13.2‰, respectively, and plot the marine carbonate rocks field in a plot of δ13CPDB vs. δ18OSMOW. It suggests that CO2 in the hydrothermal fluids was mainly originated from marine carbonate rocks.Abrupt lateral changes in facies and thickness, along with the existence of synsedimentary breccia’s and debris flows within the ore sequence, suggest the proximity of synsedimentary faults and tectonic activity contemporaneous with the sedimentation in the Lower Cretaceous, favorable to the formation of deposit. So this deposit formed by combination of sub-seafloor and on the seafloor processes. S, C and O isotopes, along with sedimentological, textural, mineralogical, and geochemical evidences, suggest that this deposit should be classified as a vent-proximal sub-seafloor replacement SEDEX ore deposit.

Facies analysis and paleoenvironmental reconstruction of Upper Cretaceous sequences in the eastern Para-Tethys Basin, NW Iran

Upper Cretaceous mixed carbonate-siliciclastic sequences are among the most important targets for hydrocarbon exploration in the Moghan area, located in the eastern Para-Tethys Basin. Despite of their significance, little is known about their facies characteristics and depositional environments. Detailed facies analysis and paleoenvironmental reconstruction of these sequences have been carried out in eight surface sections. Accordingly, four siliciclastic facies, eight carbonate facies and one volcanic facies have been recognized. Detailed facies descriptions and interpretations, together with the results of facies frequency analysis, standard facies models and Upper Cretaceous depositional models of Para-Tethys Basin, have been integrated and a non-rimmed carbonate platform is presented. This platform was affected by siliciclastic influx, in the form of coastal fan delta and submarine fans in the shallow- to deep-marine parts, respectively. This model is interpreted to be shallower in the central and northeastern parts of the Moghan area. Toward the southeast and southwest, this shallow platform turns into deep marine settings along steep slopes without remarkable marginal barriers.

Distinguishing gas-bearing sandstone reservoirs within mixed siliciclastic-carbonate sequences using extended elastic impedance: Nile Delta — Egypt

In the Abu Madi Formation of the Nile Delta Basin, false bright spots may be misinterpreted as being indicative of hydrocarbons due to mixed clastics and carbonates. However, rock-physics analysis of well logs in a particular prospect area where such ambiguity exists suggests that attributes derived using extended elastic impedance (EEI) inversion may help identify hydrocarbons because they better show anomalous behavior in particular directions that are readily related to pore fluids and lithology. The EEI attributes calculated from well logs correlate extremely well to lithology and fluid properties, thereby differentiating amplitude anomalies caused by gas-bearing sandstones encased in shale from similar amplitudes caused by juxtaposition of high-impedance carbonates over lower impedance water-filled sandstones. Comparing seismically derived EEI attributes to well logs from a productive well and a nonproductive well indicates that seismic inversion can successfully identify lithologies such as shales, sandstones, carbonates, and anhydrite and distinguish gas-bearing from water-bearing sandstones. The technique can thus potentially be used to better delineate and risk prospects in the area, as well as assisting exploration efforts in other locations where similar ambiguities in amplitude interpretation exist.

Upper Aptian mixed carbonate-siliciclastic sequences from Tucano Basin, Northeastern Brazil: Implications for paleogeographic reconstructions following Gondwana break-up

The evolution of the Cretaceous basins of the Brazilian northeastern hinterland was associated with the Gondwana rifting and opening of the South Atlantic Ocean. The first marine ingression in northeastern Brazil occurred in the late Aptian and was recorded as the Santana Group of the Araripe Basin, which is currently an isolated basin, located hundreds of kilometers away from the Brazilian marginal basins. Bellow the first upper Aptian marine deposits, an important section of fossiliferous limestone (Lagerstätte) was deposited and preserved in the Crato Formation transitioning upward into evaporites of the Ipubi Formation. The direction of the marine ingression is controversial, with several possibilities being suggested, mainly due to the absence of other areas of upper Aptian marine sections within the hinterland. Serra do Tonã is a sedimentary mesa with scarped edges where the upper part of the Marizal Formation crops out, displaying laminated limestones, litho- and chrono-correlated with those of the Crato Formation, is preserved. Therefore, this mixed upper Aptian section, at the North Tucano Basin (Serra do Tonã), is a unique occurrence of utmost importance to the definition of sedimentary events and paleogeographical reconstruction of northeastern Brazil during the late Aptian. A detailed stratigraphic analysis allowed the definition and characterization of two upper Aptian depositional sequences bounded by regional disconformities. Both sequences are dominantly transgressive and carbonate-siliciclastic in composition. The lower sequence comprises the basal portion of the Marizal Formation and consists of a succession of fluvial sandstones, ending on a laterally continuous thin interval (<15 m) of interbedded shales and limestones bearing exposure features and paleosols on the top. The limestones show a diversity of microfacies, including microbialites, organized in high-frequency deepening-upward cycles. The recognized sequence stratigraphic architecture resembles the lower part of the Barbalha Formation in the Araripe Basin, positioned in the same palynological zone, suggesting the correlation of the shale-carbonate interval in the Serra Tonã with the Batateira Beds in the Araripe Basin. The upper sequence also exhibits a fining upward pattern, with a vertical succession starting with sandstones and shales deposited in fluvial and deltaic environments, culminating upward in laminated limestones and lacustrine shales. The stratigraphic succession is very similar to the upper portion of the Barbalha Formation in the Araripe Basin, and the laminated limestones are lithostratigraphically classified as the Crato Formation. These limestones also comprise several microfacies, organized in a transgressive-regressive cycle with the maximum flooding surface positioned on relatively deep-water carbonates. Fluvial paleocurrent directions, which are similar to those of the Araripe Basin, suggest that both basins were part of the same continental paleodrainage, flowing to the south, where the South Atlantic proto-ocean was located. Fish fossils found in shales of the Marizal Formation, further south in the Central Tucano Basin and in the same stratigraphic interval of those of the lower sequence, were interpreted as marine forms. Indeed, some of them were considered to have Tethyan affinity, probably coming from an incipient Equatorial Atlantic gateway, supporting the interpretation based on the paleocurrents. The limestones at the top of the Serra do Tonã, which are also found in inselbergs in the Jatobá Basin, are relicts of a once extensive cover of Aptian carbonate deposits, now restricted because of uplifting and erosion events from the Late Cretaceous to the Cenozoic.

Facies Interpretation, Depositional Environment and Sequence Stratigraphy of the Sartakht Formation in the Bakhshi Section, Located in Kalmard Block, East-Central Iran

Permian rocks in Kalmard block are recognized with Khan Group, enjoying various characteristics in different outcrops. This group is made up of three informal formations, namely Chili, Sartakht and Hermez. Middle Permian deposits (Sartakht formation) are composed chiefly of sandstone and carbonate rocks. This formation is composed of 58.6 m sandstone and dolomitic limestone in the Bakhshi section. Lower Permian carbonate deposits (Chili formation) unconformably underlie this formation while lateritic paleosols of upper Permian (Hermez formation) are depicted overlying an erosional unconformity above this formation. According to lithologic and microscopic investigations, the deposits of Sartakht formation can be divided into 2 siliciclastic petrofacies and 12 carbonate microfacies. Field observations, along with microscopic examinations, have resulted in identifying tidal flat, lagoon, shoal and open marine environments in the rocks of the studied formation. Vertical changes of microfacies and depth variation curve point to the high thickness of the microfacies of lagoon and shoal environments and low thickness of the microfacies of tidal flat and open marine environments. The carbonate-siliciclastic sequence of Sartakht formation is made up of a third-order depositional sequence, separated from carbonate depositions of lower Permian (Chili formation) and lateritic paleosols of upper Permian by type 1 sequence boundary (SB1). Siliciclastic deposits include LST system tract, and carbonate microfacies involve TST and HST system tracts, separated from each other by MFS. Sartakht formation rocks in Bakhshi section are deposited in a low-angle homoclinal ramp, mostly in the inner ramp, located in the south of Paleotethys Ocean. The depositional sequence identified in Sartakht formation points to the age of middle Permian, conforming to middle Absaroka II supersequence. The upper erosional boundary between Sartakht and Hermez formations conforms to the global-scale sea level fall.

Exploring for unconventional hydrocarbon plays in the Glyde Basin, Northern Territory, using FALCON® airborne gravity gradiometry (AGG) data

A FALCON® AGG and magnetic survey in the Glyde Sub-basin aims to define the structural pattern to identify unconventional hydrocarbon plays. The survey area consists of variable thick fluvial and lacustrine to shallow marginal marine carbonate-siliciclastic sequences and lesser volcanic rocks. The dominant tectonic feature identified on the AGG data is the Emu Fault Zone: a major structure of the central-southern part of the McArthur Basin. Seismic and surface geology suggest its overall sub-vertical strike-slip nature with positive flower structure geometry. In the north, a regional size pop-up structure, reverse, transpressional and strike-slip faults, and abundant synclines and anticlines identified in the AGG data indicate sinistral transpression. In the south, a transtensional segment of the Emu Fault Zone formed several regularly oriented, fault controlled depocentres. A geological cross-section across the Glyde Sub-basin supported by gravity modelling indicates a system of inverted transtensional faults. Offsets and bifurcations are common, forming local-scale transtensional or transpressional areas. The relationship between dolomitic carbonaceous siltstone, fault-related hydrothermal dolomite (HTD), and the tectonic pattern strongly suggests that the Emu Fault Zone controlled fluid migration and fault-related HTD deposition. Brecciated HTD reservoirs are best developed where a combination of strike-slip movement and extension allowed dolomitising and porosity-generating fluids to migrate along fracture networks, especially in transtensional pull-apart structures, and along the principal faults bounding elevated parts of the basement. This is consistent with the results of the Glyde–1 ST1 exploration well, which drilled 122 m of gas charged dolomitic breccia.

전기고생대 태백산분지 영월층군의 순차층서 연구를 통한 고지리적 추론

전기 고생대 태백산분지 영월층군은 탄산염-규산쇄설성 퇴적암 복합체로서 하부로부터 삼방산층, 마차리층, 와곡층, 문곡층, 영흥층으로 이루어져있다. 영월층군에 대한 순차층서학적 분석에 따르면 중기 캠브리아기에 일어난 범람에 의해 최하부의 규산쇄설성 사질 퇴적암이 우세한 삼방산층이 퇴적되었다. 이어지는 후중기 캠브리아기 ~ 전후기 캠브리아기에 지속적으로 발생한 빠른 해수면 상승으로 마차리층 하부에는 셰일, 입자암, 각력암층을 협재한 사면 혹은 심부 램프 시퀀스가 형성되었다. 후기 캠브리아기 동안 지속된 해수면 상승은 실질적인 퇴적가능공간을 창출하였고, 조하대 환경에 탄산염 퇴적물 공장이 만들어졌으며, 탄산염 대지에는 마차리층을 구성하는 탄산염암이 우세한 조하대 시퀀스가 형성되었다. 마차리층 상부의 와곡층은 후후기 캠브리아기의 완만한 해수면 상승국면에서 만들어진 탄산염 램프 시퀀스로 해석되며, 퇴적 당시에는 리본 탄산염암과 탄산염 역암을 포함하는 이회암으로 구성되었던 것으로 보인다. 와곡층은 퇴적직후에 일차적으로 캠브리아기와 오르도비스기 사이의 해수면 하강국면에서 불안전 백운암화 과정을 거치고, 후에 심부 매몰 속성환경에서 광범위한 백운암화 작용을 받은 것으로 해석된다. 전기 오르도비스기에도 세계적인 해수면 상승과 해침은 지속되었으며, 영월층군의 조하대 램프 퇴적환경은 그대로 유지되어 탄산염 역암층을 협재하는 석회이암과 이회암이 교호하는 전형적인 램프 시퀀스인 문곡층이 형성되었다. 문곡층은 중기 오르도비스기에 퇴적된 것으로 알려진 영흥층에 덮여 있다. 영흥층은 주로 윤회층리를 보이는 조석대지 탄산염암으로 이루어져 있으며, 문곡층의 최상부에서 조하대 퇴적환경이 영흥층의 조석대지 퇴적환경으로 변화한다. 세계적 1차 규모 순차 경계면인 소크(Sauk)와 티피카누(Tippecanoe) 시퀀스의 경계는 영흥층 중부에서 관찰되는 최소퇴적가능공간 부근에서 인지된다. 중기 오르도비스기 초기의 세계적 해수면 하강과 이어지는 해수면의 급격한 상승은 영흥층의 전반적인 상향 천해화 윤회층의 전진퇴적체를 형성하였다. 영월층군이 퇴적된 영월 탄산염 대지의 상대적 해수면 변동곡선을 복원해 보면 같은 태백산 분지의 태백층군이 퇴적된 태백 탄산염 대지의 해수면 변동 곡선과 유사함을 확인할 수 있다. 이것은 두 개의 탄산염 대지가 유사한 조 구조적 운동 역사를 갖는다는 것을 의미하며, 이러한 유사성은 영월층군이 형성된 영월 탄산염 대지가 비록 태백층군이 퇴적된 태백 탄산염 대지와 상이한 퇴적시스템을 갖기는 하지만 상대적으로 가까운 지역에 속해 있었음을 암시한다. 퇴적층서 분석결과에 따르면 영월 탄산염 대지는 태백 탄산염 대지에 비해 상대적으로 열린 천해 환경이었을 것으로 추측된다. 고생대 후기와 중생대 전기에 걸쳐 발생한 북중국지괴와 남중국지괴의 충돌 시기에 영월 탄산염 대지와 태백 탄산염 대지가 복잡한 이동과정을 거쳐 현재의 태백산 분지에 모이게 된 것으로 해석된다. The Yeongweol Group is a Lower Paleozoic mixed carbonate-siliciclastic sequence in the Taebaeksan Basin of Korea, and consists of five lithologic formations: Sambangsan, Machari, Wagok, Mungok, and Yeongheung in ascending order. Sequence stratigraphic interpretation of the group indicates that initial flooding in the Yeongweol area of the Taebaeksan Basin resulted in basal siliciclastic-dominated sequences of the Sambangsan Formation during the Middle Cambrian. The accelerated sea-level rise in the late Middle to early Late Cambrian generated a mixed carbonate-siliciclastic slope or deep ramp sequence of shale, grainstone and breccia intercalations, representing the lower part of the Machari Formation. The continued rise of sea level in the Late Cambrian made substantial accommodation space and activated subtidal carbonate factory, forming carbonate-dominated subtidal platform sequence in the middle and upper parts of the Machari Formation. The overlying Wagok Formation might originally be a ramp carbonate sequence of subtidal ribbon carbonates and marls with conglomerates, deposited during the normal rise of relative sea level in the late Late Cambrian. The formation was affected by unstable dolomitization shortly after the deposition during the relative sea-level fall in the latest Cambrian or earliest Ordovician. Subsequently, it was extensively dolomitized under the deep burial diagenetic condition. During the Early Ordovician (Tremadocian), global transgression (viz. Sauk) was continued, and subtidal ramp deposition was sustained in the Yeongweol platform, forming the Mungok Formation. The formation is overlain by the peritidal carbonates of the Yeongheung Formation, and is stacked by cyclic sedimentation during the Early to Middle Ordovician (Arenigian to Caradocian). The lithologic change from subtidal ramp to peritidal facies is preserved at the uppermost part of the Mungok Formation. The transition between Sauk and Tippecanoe sequences is recognized within the middle part of the Yeongheung Formation as a minimum accommodation zone. The global eustatic fall in the earliest Middle Ordovician and the ensuing rise of relative sea level during the Darrwillian to Caradocian produced broadly-prograding peritidal carbonates of shallowing-upward cyclic successions within the Yeongheung Formation. The reconstructed relative sea-level curve of the Yeongweol platform is very similar to that of the Taebaek platform. This reveals that the Yeongweol platform experienced same tectonic movements with the Taebaek platform, and consequently that both platform sequences might be located in a body or somewhere separately in the margin of the North China platform. The significant differences in lithologic and stratigraphic successions imply that the Yeongweol platform was much far from the Taebaek platform and not associated with the Taebaek platform as a single depositional system. The Yeongweol platform was probably located in relatively open shallow marine environments, whereas the Taebaek platform was a part of the restricted embayments. During the late Paleozoic to early Mesozoic amalgamations of the Korean massifs, the Yeongweol platform was probably pushed against the Taebaek platform by the complex movement, forming fragmented platform sequences of the Taebaeksan Basin.

Cambrian stratigraphy of the North China Platform: revisiting principal sections in Shandong Province, China

The Cambrian succession in the North China Platform comprises a mixed carbonate-siliciclastic sequence, superbly exposed in the southern part of Shandong Province, China. In order to refine the lithostratigraphy of the Cambrian succession, this paper presents detailed sedimentary logs of outcrop sections in the Jinan, Laiwu, Jining, and Linyi areas. The entire succession consists of six lithologic units: Liguan, Zhushadong, Mantou, Zhangxia, Gushan, and Chaomidian formations in ascending order. The upper boundary of the Zhushadong Formation is refined as the base of the first purple mudstone bed of the Mantou Formation. The Mantou Formation is, in turn, bounded at the top by a thick oolitic grainstone bed of the Zhangxia Formation. The upper boundary of the Gushan Formation is placed at the base of a distinct bioclastic grainstone bed of the Chaomidian Formation. The constituent members of the Zhushadong, Mantou, and Chaomidian formations are also refined. Seventeen trilobite biozones are recognized, representing the Cambrian Series 2 to the Furongian.