Calcareous Claystone Research Articles

Data-Driven Classification and Logging Prediction of Mudrock Lithofacies Using Machine Learning: Shale Oil Reservoirs in the Eocene Shahejie Formation, Bonan Sag, Bohai Bay Basin, Eastern China

As the world’s energy demand continues to expand, shale oil has a substantial influence on the global energy reserves. The third submember of the Mbr 3 of the Shahejie Fm, characterized by complicated mudrock lithofacies, is one of the significant shale oil enrichment intervals of the Bohai Bay Basin. The classification and identification of lithofacies are key to shale oil exploration and development. However, the efficiency and reliability of lithofacies identification results can be compromised by qualitative classification resulting from an incomplete workflow. To address this issue, a comprehensive technical workflow for mudrock lithofacies classification and logging prediction was designed based on machine learning. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) were conducted to realize the automatic classification of lithofacies, which can classify according to the internal relationship of the data without the disturbance of human factors and provide an accurate lithofacies result in a much shorter time. The PCA and HCA results showed that the third submember can be split into five lithofacies: massive argillaceous limestone lithofacies (MAL), laminated calcareous claystone lithofacies (LCC), intermittent lamellar argillaceous limestone lithofacies (ILAL), continuous lamellar argillaceous limestone lithofacies (CLAL), and laminated mixed shale lithofacies (LMS). Then, random forest (RF) was performed to establish the identification model for each of the lithofacies and the obtained model is optimized by grid search (GS) and K-fold cross validation (KCV), which could then be used to predict the lithofacies of the non-coring section, and the three validation methods showed that the accuracy of the GS–KCV–RF model were all above 93%. It is possible to further enhance the performance of the models by resampling, incorporating domain knowledge, and utilizing the mechanism of attention. Our method solves the problems of the subjective and time-consuming manual interpretation of lithofacies classification and the insufficient generalization ability of machine-learning methods in the previous works on lithofacies prediction research, and the accuracy of the model for mudrocks lithofacies prediction is also greatly improved. The lithofacies machine-learning workflow introduced in this study has the potential to be applied in the Bohai Bay Basin and comparable reservoirs to enhance exploration efficiency and reduce economic costs.

Novel insights about petroleum systems from source and reservoir rock characterization, Cretaceous Deposits, Babouri-Figuil Basin, Northern Cameroon

The Babouri-Figuil Basin (BFB) is a frontier basin for petroleum in Cameroon. It belongs to the series of Cretaceous rift basins of the West and Central Rift System (WCARS), the origin of which is related to the opening of the South Atlantic. Within the same rift system, commercial hydrocarbon accumulations have been discovered in Chad, Sudan, Niger and, more recently, in Nigeria (Gongola Basin). The study of the geology of the BFB just recently received considerable attention, mainly because of its presumed hydrocarbon potential. In the pursuit of researching possible petroleum systems in the BFB, the current study provides a first look into the characterization of source and reservoir rock and its integration into a 2D lithostratigraphic model. The study was solely based on outcrop samples. Black shale and massive claystone are good to excellent hydrocarbon source rocks [e.g., up to 38 wt% total organic carbon (TOC), up to 943 mg/g hydrogen index, up to 85 m thickness, up to 20–30 km lateral extension], with moderate to high values of extractable organic matter (e.g., >10,000 ppm). Calcareous claystone, on the other hand, are poor source rocks [e.g., <0.20 wt% TOC]. The samples are thermally immature, except for those located close to volcanic intrusion at Golombe that have reached the threshold for oil generation (Tmax >435 °C, production index >0.1). The petrographic analysis of sandstone revealed that they are fine-grained to coarse-grained, poorly to moderately sorted, texturally and compositionally immature to submature, subarkosic to arkosic arenites. The main diagenetic processes that affected sandstones are as follows: moderate to intense compaction characterized by the development of long, concavo-convex, and sutured contacts between grains; cementation through calcite, iron oxide, and quartz cements; alteration of mica and feldspar grains; partial to complete dissolution of feldspar, mica, amphibole grains, and calcite cement; and the replacement of feldspar and mica grains by clay minerals. Alteration and dissolution increase the porosity of sandstone through the creation of secondary pores. However, mechanical compaction through the development of a pseudomatrix and cementation as pore-filling materials have significantly reduced the quality of sandstone beds as conventional petroleum reservoirs. Hence, the best reservoir-quality sandstones in the basin are generally located in the upper portion of the basin in terms of its lithostratigraphic model. They are the cleanest sandstones with the smallest amount of cement and the lowest ductile grain content (pseudomatrix), with a thickness that varies from 3 m to 120 m and a lateral extension of 20 km. The lithostratigraphic model of the basin is characterized by an extensive lacustrine environment that provided a thick sequence of organic-rich formations; sand deposited as extensive reservoirs sandwiched between shale/claystone beds; the development of stratigraphic traps through lateral facies change; and the widespread deposition of lacustrine and floodplain claystone that provide regional seals. The similarities between the Babouri-Figuil Basin and proven petroleum systems in other WCARS rift basins suggest that the basin may host at least one petroleum system where actively generating source rocks are present.

The Brehov volcanogenic and stratabound base metal and gold deposit (Eastern Slovakia): Position and genetic relations in the Internal Carpathian–Alpine Cenozoic metallogenetic belt

The Brehov ore deposit, near the village of Brehov, is located in the East Slovakian Basin 15 km south-east of the Trebišov town. A progressive development of Middle Badenian to Middle Sarmatian sedimentation and volcanism in the Brehov hydrothermally mineralized area is manifested in the article. The volcanogenic, stratabound, polymetallic (Zn, Pb, Cu) and gold sulphide deposit within the Brehov–Sirník interpreted resurgent caldera occurs in the volcanosedimentary sequence. Mineralization in the deposit, consisting of the fine-grained aggregate of sulphides, is interpreted to be of shallow water origin. It is placed mostly in the rhyodacite volcaniclastics and partly in calcareous claystone, altered and brecciated. Three types of ores – the stringer (keiko), yellow and black ore – were distinguished near the Brehov village on the basis of ore composition, being formed by successive mineralization stages. The stringer ore, consisting of the sulphide minerals, dominates in the quartz-dioritic (siliceous) rocks. The yellow ore is represented primarily by pyrite, but contains also minor chalcopyrite and quartz. The black ore is an intimate mixture of sphalerite, galena, sometimes barite and minor pyrite and chalcopyrite. Tetrahedrite and marcasite occur sporadically. The ore in all cases is fine-grained. The ore bodies are almost everywhere enclosed by clay, composed of montmorillonite, sericite and chlorite. The ore bearing fluids were probably colloids of hydrothermal origin. Textures are considered to indicate that the colloids replaced the country rocks, rather than merely filled open spaces in the rhyodacite pyroclastics and epiclastics. Despite the precious metals (mostly gold) are economically important commodity in many Kuroko type deposits, in the Brehov ore deposit they occur in volumetrically minor amounts. Visible gold is present as inclusions of native gold in major sulphide minerals, whereas silver occurs in Ag-sulphides and sulphosalts, e.g. tetrahedrite and ferberite.

Nannofossil Biostratigraphy of Elat Formation, Kei Besar Island, Southeast Maluku

The first detailed mapping and recording of nannofossils was carried out on Kei Besar Island, Southeast Maluku. This area is mostly composed of the Elat Formation, which is mainly composed by intercalated calcarenite and calcareous claystone. Total 48 samples were taken on three measured sections (Hollat, Ngurdu and Mata Hollat). Samples were prepared using smear slide method, then observed under a polarized microscope at 1000 x magnification. Total 48 species of nannofossils were identified. Several index species were selected to define biostratigraphic zones based on their first and last occurrence. Three zones of nannofossils are identified, starting with the oldest those are: Reticulofenestra umbilica zone (NP16; 43.06 to 38.7 million years ago), Helicosphaera compacta zone (NP17; 38.7 to 37.9 Mya) and Helicosphaera eupratis zone (NP18-NP19; 37.9-36.8 Mya). It can be concluded that Elat Formation is deposited in Middle to Late Eocene (43.06 to 35.4 Mya). The Middle-Late Eocene boundary can be clearly defined based on nannofossils assemblages in the section of the Kei Besar Island.

Stratigrafie sedimentů doprovázejících vulkanity těšínitové asociace u Bruzovic

The rocks of the teschenite association form smaller scattered occurrences in the upper part of the Hradiště Formation of the Silesian Unit. The age of the igneous rocks corresponds to Berriasian to Barremian (Menčík et al. 1983). The teschenite association includes a relatively heterogeneous group of rocks with variable abundances of felsic and mafic minerals. Three outcrops of the teschenite association rocks (both typical teschenites and strongly altered ultramafic types) embedded in non- volcanic sediments were studied in the bed of the Pazderůvka River between Bruzovice and Žermanice (Bz1, Bz2, and Bz3; Fig. 1). At Bz1 and Bz2 outcrops, bodies of teschenite are hosted by sedimentary rocks affected by caustic metamorphism. The volcanic rocks at Bz3 are represented by strongly altered, markedly clayey dark green rocks with a macroscopically apparent high proportion of dark micas and pyroxenes. The bedrock of the outcrop is composed of calcareous claystone, limestone, and sandy limestone, likely representing Těšín Limestones. The phase composition of the volcanic rocks from the Bz1 and Bz2 localities is to a certain extent similar, which indicates that the same body was observed at both localities. Macroscopically, the rocks are medium- to coarse-grained with a variable proportion of dark minerals, indicating internal differentiation of the body. The very high proportions of natrolite (up to about 35 %, up to 54 % in the leucocratic type) are particularly notable. The volcanic rocks from the Bz3 locality are quite different. Their composition corresponds to strongly argilitised mafic rocks (and their volcaniclastic derivates), containing more than 50 % of clay component with a dominance of smectite, as documented by X-ray diffraction data. As for the primary components, pyroxene, biotite, plagioclase, and accessory amphibole were detected. Calcareous dinoflagellate cysts Cadosina semiradiata fusca, Colomisphaera vogleri, and Cadosina semiradiata representing the lower Valanginian to upper Barremian were identified in polished sections of the calcareous rock. Non-calcareous dinoflagellate cysts in Bz1 and Bz2 samples are almost black, reflecting the strong thermal effects of the teschenite intrusion at the contact with with the sediments. The dinoflagellate cysts in the Bz3 outcrop are very well preserved with no evidence of thermal alteration. The overlying body of the volcanic rock can be therefore considered as effusive. Circulodinium vermiculatum, Cymososphaeridium validum, Muderongia tabulata, Oligosphaeridium albertense, Oligosphaeridium asterigerum, Pseudoceratium pelliferum, Systematophora palmula from the Bz3 outcrop can be considered as stratigraphically significant. Based on the first occurrence of C. validum and the last occurrences of Circulodinium vermiculatum and Systematophora palmula, Leereveld (1995) delineates a dinoflagellate zone of Cymososphaeridium validum which represents the upper Valanginian to the lowermost Hauterivian. The composition of palynofacies, in which palynomorphs predominate, is characteristic of a distal well-oxygenated shelf.

Latest Cretaceous and Paleocene biostratigraphy and paleogeography of northern Zealandia, IODP Site U1509, New Caledonia Trough, southwest Pacific

ABSTRACT IODP Site U1509 (Expedition 371), New Caledonia Trough, provides a rare latest Cretaceous–Paleocene record from offshore northern Zealandia. We present new palynomorph and benthic foraminiferal assemblage data that show a transition from a latest Cretaceous vegetated sediment source region to a fully oceanic environment in the Paleocene. Latest Cretaceous (c. 68–66 Ma) non-calcareous claystone was deposited in upper bathyal paleodepths, with abundant plant microfossils that were likely transported in a northwest direction through the Aotea Basin region. A 2–3 Myr unconformity is identified at the Cretaceous/Paleogene boundary. Middle early–late Paleocene (c. 63.5–56 Ma) calcareous claystone shows evidence of deepening, deposited in middle bathyal or deeper paleodepths, and terrestrial input is minor. This latest Cretaceous to Paleocene deepening trend is consistent with inferred evolution of the Aotea and Northland basins further to the east, and other sparse northern Zealandia records, which show a common pattern of post-rift transgression consistent with long-term subsidence. Site U1509 allows for biogeographic extension and modification of the New Zealand Paleocene dinoflagellate zonation, description of a new dinoflagellate and pollen species, better inference of regional paleogeography, and may provide insights into the onset of western Pacific subduction initiation.

Changes in benthic microfossil assemblages before, during and after the early Toarcian biotic crisis in the Portland-Wight Basin (Kerr McGee 97/12-1 well, offshore southern England)

The uppermost Pliensbachian (Margaritatus Zone) - upper Toarcian (Pseudoradiosa Zone) sediments (silty and variably calcareous claystones, marlstones, silty sandstones, and sandy limestones) recovered from the Kerr McGee 97/12–1 well were deposited in the Portland-Wight Basin. The foraminiferal assemblages of these sedimentary succession are dominated by calcareous forms of the suborder Lagenina and locally Robertinina. Three main ecostratigraphic intervals have been identified, based on changes in the foraminiferal and ostracod assemblages that occurred before, during and after the sedimentation of the earliest Toarcian black, organic-rich, non-calcareous claystones. The ecostratigraphic interval I (upper Pliensbachian) is characterized by a diverse assemblage with specialist, opportunist and intermediate forms, but dominated by Lenticulina muensteri. The ostracods in this interval are dominated by Metacopina. This assemblage is interpreted to represent a phase that pre-dates the biotic crisis. The ecostratigraphic interval II, related to the deposition of the lower Toarcian black claystones, shows an abrupt increase in the opportunist form Reinholdella (91%) with very low diversity and a high abundance of specimens, mainly Reinholdella macfadyeni and R. dreheri. In this interval the ostracods Metacopina dissapear. The top of the black claystones is almost barren of foraminifera and ostracods (only two specimens recorded of Trochammina eoparva). This second interval represents the biotic crisis phase related to the Jenkyns Event. The ecostratigraphic interval III (upper part of the lower Toarcian through to the upper Toarcian) is characterized by an increase in the diversity and abundance of foraminifera, as well as the dominance of Lenticulina. The ostracod assemblage shows a turnover with respect to the upper Pliensbachian assemblage with dominance of Cytherocopina, Platycopina and Bairdiocopina. This assemblage is interpreted to reflect the increased availability of oxygen from deep-infaunal to epifaunal microhabitats and the return to normal conditions at the sea floor. The incidence of the global warming related to the Jenkyns Event in the biotic crisis is also considered. Ecostratigraphic fluctuations in microfossil assemblages from the Kerr McGee 97/12–1 well across the T-OAE event are comparable with those described from the Mochras Borehole (west Wales), the Lusitanian Basin (Portugal), and the Atlassic Basin (North Africa).

Electrical properties of sedimentary microfacies and depositional environment deduced from core analysis of the syn-rift sediments, Northwestern shore of Gulf of Suez, Egypt

The present work is based mainly on core rock samples investigation of CSS-38 well, northwest of Gulf of Suez, Egypt. The cores cover the whole the South Gharib Formation (upper Miocene). The stratigraphic succession of the Gulf of Suez includes three mega units, recorded from old to young as pre-rift, syn-rift, and post-rift sediments. The syn-rift sediments from old to young are Nukhul, Rudies, Kareem, Belayim Equivalent, South Gharib, and Zeit. The core samples reveal that South Gharib Formation is not a good reservoir. It consists of sulphate-rich beds with few carbonates intercalation, laminated dolomitic limestone and marly limestone. Patchy oil staining is detected in the thin streaks and basal laminated dolomitic limestone. The microfacies are found to be evaporites, carbonates, calcareous marl, and calcareous claystone. Evaporites microfacies are considered the most frequent microfacies. Gypsum and anhydrite are generally represented by nearly equal distribution with parallel variation in content. The core samples differentiated South Gharib Formation into seven zones of depositional environment; they are arranged from base to top as shallow sub-tidal, shallow lagoonal, intertidal, shallow lagoonal, intertidal, shallow lagoonal, and supra-tidal Sabkha cycles. Some samples from South Gharib Formation were measured electrically. We will try to highlight interrelations between electrical (conductivity, permittivity, and impedance) and petrophysical properties with petrography and mineral composition. Electrical properties were measured with a frequency range from 10 Hz to 100 kHz. The dielectric constant decreases with increasing frequency and increases with increasing conductor content (up to percolation threshold). The conductivity increases with the increase in conductor content and with increasing frequency. Many parameters can combine to affect electrical properties. The present research is a trial to relate the surface to subsurface measurements.

Depositional Environment of Nampol Formation and Its Implications as Potential Source Rocks in the Sumbermanjing Area, South Malang

The decline in oil and gas production inevitably forces explorationists to be more creative in finding new potential reserves. Nampol Formation is expected to have the potential to become hydrocarbon source rocks due to their organic matter content. However, the depositional environment of Nampol Formation has not been thoroughly researched. The focus of this research is to reveal the depositional environment of Nampol Formation that may lead to a suitable condition for the deposition of potential source rocks layers. Detailed geological mapping in several selected traverses and micropaleontological analysis was carried out to determine lithological characteristics and depositional environment of Nampol Formation. The microfacies approach was also used to determine the carbonate facies zone of this unit. Source rock samples taken from outcrops on the surface then were analysed to evaluate organic matter content, type of kerogen thermal maturity, and its potential to produce hydrocarbons. The Nampol clastic limestone unit in the study area is composed of clastic limestone with intercalation of black shale, claystone, siltstone, sandstone and calcareous claystone. The microfacies approach shows that the wackestone and mudstone belong to the SMF 15-M (Ooid wackestone) and SMF 23 (Non-fossiliferous mudstone) which can be interpreted belong to Facies Zone (FZ) 8 in the Flügel carbonate facies model (2004). This zone is located in the Restricted interior platform or platform environment with closed water circulation which is situated behind the barrier reef (back reef). Based on the results of geochemical analysis, source rock samples from Nampol Formation can be considered as a gas prone source rock, with type III kerogen, total organic content ranging from 3.48 – 26.18 wt%, po-sess the potential to produce

Petrographic and diagenetic study of siliciclastic Jurassic sediments from the northeastern margin of Africa: Implication for reservoir quality

Northeastern Egypt is one of the most important exposures in the Middle East and North Africa to study Jurassic facies. The present study analyzes 130 thin sections from a subsurface well (Well X) in the northern Gulf of Suez, and from two surface sections at Gebel Maghara (north Sinai) and Khashm Elgalala (North Eastern Desert), Egypt for petrographic and lithofacies analyses. In the present study, detailed petrographic analysis is used as a tool to better understand the diagenetic history of the Early to Late Jurassic siliciclastic sediments; the identified diagenetic elements are inferred concerning the reservoir quality. For this, sidewall cores, scanning electron microscopy and thin sections are used to detail detrital and authigenic mineralogy; these are then used to infer the depositional framework, factors controlling reservoir characteristics, and the operating diagenetic processes. The inferred depositional paleoenvironment is a prograding and retrograding linear siliciclastic shoreline within a shallow marine carbonate platform with coal swamps and occasional cross-cutting rivers. Diagenesis, petrographical characteristics and depositional conditions are the main factors controlling continental and marine reservoir architecture. The diagenetic processes affecting siliciclastic sedimentation are near the surface, shallow to intermediate burial, and deep burial cycles with different pore fluid filling at each stage. The siliciclastic sediments have been categorized into seven depositional lithofacies - calcareous claystone (S1a), carbonaceous claystone (S1b), siltstone (S2), planar and trough cross-bedded sandstone (S3), coarse, well rounded, large scale trough cross-bedded sandstone (S4), ooid sandstone (S5) and coal (C). Grain compaction, various phases of syntaxial quartz overgrowth, carbonate cementation and replacement, creation of dissolution porosity, and clay authigenesis are the most important diagenetic processes that have affected the siliciclastic continental and marine sediments. Many different diagenetic events, both destructive and constructive, have modified porosity. The destructive events include compaction and cementation (of silica, kaolinite, ferroan dolomite, Illite, and anhydride). The constructive events include silica cementation, feldspar dissolution, and ferroan dolomite dissolution. Additionally, present data also suggest that the continental sandstones have excellent reservoir quality whereas the marine sandstones have good to very good reservoir potential. Despite the excellent reservoir quality of the continental sandstone lithofacies, the effective stratigraphic seal is leaking. The interbedded calcareous and carbonaceous claystone lithofacies may provide an excellent stratigraphic seal for these siliciclastic reservoirs. Based on these analyses, a model is proposed that can be used as a template for subsurface Jurassic reservoir characterization and reservoir discrimination.

Kinetics of hydrocarbon generation from the marine Ordovician Goldwyer Formation, Canning Basin, Western Australia

Pyrolysis and bulk kinetic studies were used to investigate the hydrocarbon generation potential and source rock organofacies variability of marine organic-rich rocks from the Middle Ordovician (Darriwilian) Goldwyer Formation in the Canning Basin, Western Australia. Rock-Eval pyrolysis results for the analysed immature to mid-mature calcareous mudstones imply that the upper Goldwyer Sequence I contains oil-prone type I kerogen, while the lower Goldwyer Sequence III contains type II/III oil- and gas-prone kerogen. This is supported by pyrolysis gas chromatography (Py-GC) results that show the presence of homogenous organofacies in the Goldwyer Sequence I having aliphatic molecular signatures, possibly due to selective preservation of lipids derived from Gloeocapsomorpha prisca (G. prisca). The heterogeneous organofacies of the Goldwyer Sequence III contain aromatic moieties in similar abundance to the aliphatic compounds. The calcareous claystones of the Goldwyer Sequence I have the capacity to generate low wax paraffinic oil, whereas the Goldwyer Sequence III has potential for paraffinic-naphthenic-aromatic (P-N-A) low wax oils, gas, and condensate.The temperature for hydrocarbon generation for the type I kerogen, assuming a constant geological heating rate of 3 °C/my, is estimated to occur over a narrow interval between 145 °C and 170 °C for the Goldwyer Sequence I samples. Generation from the type II/III kerogen occurs from 100 °C to 160 °C in the Goldwyer Sequence III samples which are significantly less thermally stable than the Goldwyer Sequence I samples. The kinetic results for both sequences were used in standard thermal and burial history plots to evaluate their transformation ratio and hydrocarbon generative potential. This provided basin-specific kinetic inputs for burial history modelling and better constraint on kerogen transformation and hydrocarbon generation on the Broome Platform.

Nannofossil Distribution and Age of Kendeng Zone In Kalibeng River Section of Kedungringin, Plandaan Area, Jombang, East Java

DOI:10.17014/ijog.7.1.15-24This study was carried out at Kalibeng River Section, in Kedungringin area, a site with lithology suitable for a nannoplankton research. Methods used in this research include performing a stratigraphical measurement section at the field, collecting thirty-two samples, and preparing the samples with the smear slide method using a polarizing microscope with 1000x magnification, and preparing several rock samples using SEM analysis. The analysis of nannofossil resulted in twelve genera and forty-three species. The identified genera are: Calcidiscus, Coccolithus, Ceratolithus, Discoaster, Gephyrocapsa, Helicosphaera, Pseudoemiliania, Reticulofenestra, Rhabdosphaera, Sphenolithus, Syracosphaera, and Umbilicosphaera. The presence of these genera indicates that Kalibeng River Section have abundant nannofossils and based on the nannofossil analysis, from older to younger, the studied stratigraphic sequence indicates a more detailed age determination as follows: the Marl Unit of Kalibeng is NN10-NN18 (Middle Miocene to Pliocene), Calcareous Sandstone Unit of Sonde is NN19-NN20 (Pliocene-Pleistocene), and Calcareous Claystone Unit of Sonde is NN20-NN21 (Pleistocene).

Biostratigraphic analysis based on planktic Foraminifera from well X in North Sumatra Basin

There have been several studies conducted in the North Sumatra Basin, but there is a scanty information related to the age of the Formations in the basin. The aim of this study is to establish the age of the Formation. A total of 128 ditch cutting samples are used for lithostratigraphy analysis, in which 33 samples were analysed further for the Foraminifera biostratigraphy study from the well X at a depth of 6090 – 9919 feet. From the result of the analysis, 38 species from 16 genera successfully identified, as well as 5 datum species, namely FO Globigerinoides sicanus, FO Orbulina suturalis, FO Globorotalia menardii, FO Globorotalia plesiotumida and FO Globorotalia tumida. From the lithology data, three (3) lithostratigraphic unit successfully identified, namely the calcareous claystone, very fine calcareous sandstone and fine calcareous sandstone unit which is estimated to be comparable to the Baong, Keutapang and Seurula Formation. From the result of the study it’s concluded that the lithostratigraphic units found in the study area have the age range of N7-N18 or the late Early Miocene to the Late Miocene.

Volcanism in The Pre-Semilir Formation at Giriloyo Region; Allegedly as Source of Kebo-Butak Formation in the Western Southern Mountains

Kebo-Butak Formation was known to be the oldest volcanic rocks limited in regional terms in the lower Baturagung Hills, Gedangsari area, Gunungkidul Regency. The main constituents of the Kebo-Butak Formation consist of intersection of volcanic-clastic rocks and calcareous sediments, locally also found basalt lava with pillow structures; which distinguished it from other volcanic rock formations in the Southern Mountains. This study aims to determine the relationship of volcanic rocks exposed in Giriloyo with the Kebo-Butak Formation in the Baturagung Hills; the chronostratigraphy and the history of volcanic activities that produced the volcanic rocks of Giriloyo. This research was approached by volcanic geological mapping using surface mapping suported by gravity anayses. From the bottom to the top of the frontier areas result volcaniclastic rocks consisting of black tuffs with several fragments of volcanic bombs with basalt composition intersecting with thin basaltic lava inserted by calcareous claystone having an age of N5-7 (Early Miocene); pyroxene-rich basalt volcanic sequence consists of thick layers of tuff with creamy-brown color intersecting with lava and breccia inserted by calcareous sandstone aged N7-8; dikes, lava and agglomerates with basaltic composition and lava and agglomerates with andesitic composition. Stratigraphically, the volcanic rocks exposed at Giriloyo correlated with the volcanic rocks exposed at Karangtalun (Wukirsari) were under the Semilir Formation, bordered with normal fault N210oE/77o, the hanging wall composed by light grey tuff of Semilir Formation. Gravity analyses found high anomalies below the Semilir Formation exposed at Karangtalun-Munthuk (east of study area) continued to below the Giriloyo area. The high anomalies were identified as the igneous/ignimbrite volcanic sequence. Descriptively and stratigraphically, the Giriloyo volcanic sequence are a part of Kebo-Butak Formation. The petrogenesis of the volcanic rocks will be discussed in further research to interpret magmatological properties, the evolving paleo-volcano, and the absolute age of the rocks.

Dinoflagellates and calpionellids of the Jurassic–Cretaceous boundary, Outer Western Carpathians (Czech Republic)

A palynological analysis and study of calpionellids and calcareous dinoflagellate cysts across the Jurassic–Cretaceous (J/K) boundary succession in the Outer Western Carpathians, Bruzovice section is presented. It is a one of few sections of the Tethyan Realm suitable for the palynological study. The lower part of the section is formed by anoxic deposits consisting of dark grey calcareous claystones. The upper part of the profile is represented by flysch-like alternation of grey limestones, spotted claystones and marlstones. The section spans the stratigraphic range from the upper Tithonian, Cadosina semiradiata calcareous dinoflagellate Zone to the lower Berriasian, calpionellid Calpionella elliptica Subzone. The J/K boundary is marked by a high abundance of small forms of Calpionella alpina. The Tithonian–Berriasian succession of non–calcareous dinoflagellates is as follows: the base of the section is correlated with the dinocyst Semiradiata Zone along with the presence of Glossodinium dimorphum. The first occurrence (FO) of Amphorula metaelliptica and the last occurrence (LO) of Prolixosphaeridium anasillum are recorded in the Cadosina semiradiata Zone followed by the Cadosina tenuis–Colomisphaera fortis Zone. An aboudance of the spherical C. alpina indicates the onset of the Calpionella Zone and the Calpionella alpina Subzone at the base of the Berriasian Stage. The FO of Diacanthum hollisteri is situated in the lower part of the Calpionella alpina Subzone. The FO of Dichadogonyaulax bensonii coincides approximately with the base of the calpionellid Remaniella ferasini Subzone, and the FO's of Muderongia longicorna, M. tabulata and Spiniferites cf. ramosus were found in the upper part of the calpionellid Remaniella ferasini Subzone. The FO of Achomosphaera neptunii was found in the lower part of the Calpionella elliptica Subzone. The work is a contribution to the ICS Berriasian Working Group.

3D seismic characterization of the polygonal fault systems and its impact on fluid flow migration: An example from the Northern Carnarvon Basin, Australia

We used high-resolution 3D seismic data and applied seismic attribute analysis to geometrically interpret and characterize the polygonal fault systems in the Paleocene-Eocene sequence of the Rankin Platform Sub-basin, Northern Carnarvon Basin, Australia. The polygonal faults were identified in a stratigraphic sequence dominated by calcareous clay with minor calcilutite at its lower section that changes to argillaceous calcilutite and calcareous claystone at its upper section. Seismic sections and time slices, through multi-attribute (coherence, energy ratio similarity, and curvature) volumes display a dense distribution of normal faults with polygonal shape and small throws in the plain view. The polygonal fault systems (PFS) exist in two well-defined tiers, which can act as pathways for the fluid flow migration. The first tier coincides with the upper section (Early-Middle Eocene Wilcox Formation) with a few faults and the second one coincides with the lower section of the sequence (Paleocene Lambert and Dockrell formations) with a greater number of faults. Proper understanding and interpretation of the polygonal fault systems as non-tectonic extensional faults are a critical issue in assessing the migration pathway for hydrocarbon (gas) through the Paleocene-Eocene sequence in the Northern Carnarvon Basin, and elsewhere in the world.

Petrographic Characteristics and Depositional Environment Evolution of Middle Miocene Sediments in the Thien Ung - Mang Cau Structure of Nam Con Son Basin

DOI: 10.17014/ijog.4.3.143-157This paper introduces the petrographic characteristics and depositional environment of Middle Miocene rocks of the Thien Ung - Mang Cau structure in the central area of Nam Con Son Basin based on the results of analyzing thin sections and structural characteristics of core samples. Middle Miocene sedimentary rocks in the studied area can be divided into three groups: (1) Group of terrigenous rocks comprising greywacke sandstone, arkosic sandstone, lithic-quartz sandstone, greywacke-lithic sandstone, oligomictic siltstone, and bitumenous claystone; (2) Group of carbonate rocks comprising dolomitic limestone and bituminous limestone; (3) Mixed group comprising calcareous sandstone, calcarinate sandstone, arenaceous limestone, calcareous claystone, calcareous silty claystone, dolomitic limestone containing silt, and bitumen. The depositional environment is expressed through petrographic characteristics and structure of the sedimentary rocks in core samples. The greywacke and arkosic sandstones are of medium grain size, poor sorting and roundness, and siliceous cement characterizing the alluvial and estuarine fan environment expressed by massive structure of core samples. The mixed calcareous limestone, arenaceous dolomitic limestone, and calcareous and bituminous clayey siltstone in the core samples are of turbulent flow structure characterizing shallow bay environment with the action of bottom currents. The dolomitic limestones are of relatively homogeneous, of microgranular and fine-granular texture, precipitated in a weakly reducing, semi-closed, and relatively calm bay environment.

Nanggulan Formation and Its Problem As a Basement in Kulonprogo Basin, Yogyakarta

DOI: 10.17014/ijog.4.2.71-80Nanggulan Formation consists of the oldest clastic rock sequence exposed in Kulonprogo area, Yogyakarta. This paper discusses the position of Nanggulan Formation as a basement. The method used in this research is surface and subsurface investigations based on gravity surveys. The rock assemblage is exposed and distributed partly in the east flank of Kulonprogo Mountains with weak undulated morphology. The rock sequence is composed of sand to clay grain sizes such as sandstone, quartz sandstone, calcareous sandstone, claystone, fossiliferous claystone, calcareous claystone, siltstone, and coal seam intercalations. The total thickness of the sequence is less than 200 m. Based on the fossil and palynology investigations, previous investigators concluded the age of the rock was Eocene to Middle Miocene. The geological structures developed in the rocks are the lithological stratification, fractures, folding, and faulting. The subsurface interpretation based on gravity data revealed the rock was located under the andesite breccias with 2.44 g/cc density. The density of the rock sequence was 2.63 g/cc. The gravity interpretation shows a strong indication that Nanggulan Formation underlies the andesitic breccias presumably associated with Old Andesite Formation exposed in Kulonprogo Mountains. The limited distribution, the thickness, and the closed environmental deposition of Nanggulan Formation found in the present investigation raised problems on the position of the formation as the basement of Old Andesite Formation occurring in the Kulonprogo Mountain.

Siliciclastic input and sedimentation rate as controls on paleoecological distribution of Lower Miocene benthic carbonate producers in a fan–delta: Zagros foreland basin, Iran

This paper aimed to study Lower Miocene (Burdigalian) mixed carbonate–siliciclastic deposits within an Upper Cenozoic synorogenic conglomerate–dominated succession in north of Shalamzar in the Zagros foreland basin, Iran. The deposits are composed of nine facies: foraminiferal mudstone, silty mudstone, sandy mudstone, fossiliferous sandy mudstone, fossiliferous argillaceous mudstone, fossiliferous calclithite, coral limestone, calcareous claystone and hybrid sandstone. The facies represent a mixed carbonate– siliciclastic shelf–type fan–delta. The subenvironments of the fan–delta include muddy pro–delta, sandy delta– front, clastic proximal mouth bar and a subordinate delta plain. Siliciclastic input and sedimentation rate controlled the paleoecological distribution of different benthic carbonate–producing fauna in the fan–delta during its progradation into a shelf marine environment. Input of siliciclastic deposits and sedimentation rate limited the diversity and development of corals and controlled their colonization and growth morphologies in the sandy delta–front. Siliciclastic input (including plant materials and coal debris) and sedimentation rate controlled the trophic habitats of many gastropods and their abundance and distribution in the sandy delta– front and clastic proximal mouth bar. Also, increased siliciclastic input favored abundance of larger benthic foraminifera in most parts of the fan–delta with the exception of the muddy pro–delta.

The formation placement and palaeoenvironment of the Middle Miocene Los Atajos Member, Trinidad

The age, palaeoenvironment and formation placement of the conglomeratic Los Atajos Member of central Trinidad have long been unclear. Seven samples (four from calcareous silts near the member's base exposed by building work at the Los Atajos Community Centre, and three from the underlying calcareous claystones of the Brasso Formation) were examined for calcareous microfossils. These indicate a conformable succession encompassing an overall regression. The oldest of the claystone samples, of uppermost early Middle Miocene Globorotalia fohsi fohsi Zone age (N11), contained an upper bathyal benthic foraminiferal assemblage, while the younger claystone samples yielded abundant, shallow neritic Hanzawaia carstensi. The recovery of the planktonic foraminifera Globorotalia mayeri but absence of Globigerinoides subquadratus suggest a Middle Miocene age (Globorotalia mayeri planktonic foraminiferal Zone; N14) for both the uppermost Brasso claystone sample and the Los Atajos Member. The benthic foraminiferal fauna in the Los Atajos is dominated by Rosalina subaraucana, with subdominant Cibicides ex gr. aknerianus and C. floridanus sensu Galloway and Heminway, and common Elphidium spp. This faunal succession has a close affinity with that of parts of the Lower to Middle Miocene Brasso Formation, especially the N10 Guaracara Limestone Member and the clays on which it sits. However, the Los Atajos Member assemblage differs markedly from that of the overlying Late Miocene San José Calcareous Silt Member of the Manzanilla Formation, from which the Los Atajos is separated by an unconformity of Globorotalia menardii (N15) Zone age. On these grounds, the Los Atajos Member is here placed within the Brasso Formation. The low diversity, high dominance benthic foraminiferal fauna and the associated ostracod assemblage in the Los Atajos are indicative of inner to shallower middle neritic palaeodepths in a carbonate-prone palaeoenvironment with marine vegetation and strong current action. It is concluded that the Los Atajos was deposited prior to the Late Miocene diversion of the palaeo-Orinoco River towards Trinidad, which is marked by the Manzanilla Formation.