Clastic Turbidites Research Articles

Exploration potential of Papua New Guinea – latest multiclient 3D provides new insight into the underexplored Papuan Basin over the Papuan and Eastern Plateaus

The Gulf of Papua, part of the regional Papuan Basin, is characterised by extensional to passive margin tectonics from the Triassic to Late Oligocene and collisional tectonics from the Late Oligocene until the present day. The basin has experienced rapid clastic deposition in the late Neogene which covers an underlying extensive Miocene carbonate shelf with local bioherms and associated carbonate facies. Underlying the carbonates, an array of deformed and complex rift basins are present, with potential petroleum plays in undifferentiated Mesozoic sediments which onlap basement highs. The Miocene carbonates are the primary exploration targets, along with Pliocene clastic turbidite sediments in the Fly River Delta, Western Gulf of Papua. It is postulated that these were charged by Mesozoic marine–deltaic/terrestrial source rocks that were deposited during the opening of the Coral Sea. The offshore Papuan Basin is underexplored and the Gulf of Papua play types seen in the onshore Aure Fold Belt and offshore Fly River Platform could potentially extend offshore into the Coral Sea area. New broadband Pre‐Stack Depth Migration seismic data covering a part of the Eastern and Papuan Plateaus reveals new insights into the structural development of the region, rift basin geometries, configuration, and association of rift clastics and post-rift carbonates. Internal carbonate features can be observed on the new seismic data set, and new plays in the yet undifferentiated rift sediments of Mesozoic age are proposed in this study.

Diagenesis and petrophysics of Miocene sandstones within southern Apennines foreland, Italy

Deep-marine clastic turbidite beds commonly occur in the Southern Apennines and record the main tectonic phases associated with the structural evolution of the area. Based on variations of detrital compositions through time, the sandstone suites are traditionally subdivided into four petrofacies: (I) quartzolithic (Burdigalian-Langhian), (II) quartzofeldspatholithic (Langhian-Tortonian), (III) quartzofeldspathic (Serravallian-Tortonian), and (IV) arkose (Tortonian-Messinian) sandstones. Since the complexity and diversity of these sandstone petrofacies, we aim to investigate their major diagenetic events and relate them with their detrital modes and physical properties in order to obtain key basic information at a regional scale, to better understand the main controls on petrophysical properties in this and in similar contexts. The petrophysical analysis reveals that the quartzolithic sandstones have the lowest porosity values, with a median of 4.56%. In contrast, porosity increases to median values of 16.75% in the quartzofeldspathic and arkosic petrofacies. The median permeability values range between 4.85 and 236.38 mD, and its distribution is generally poorly correlated with any other petrophysical parameter. The most widespread diagenetic minerals observed are carbonates, clay minerals, Fe-oxides and hydroxides, and less quartz. These sandstones are characterized by high compaction, which is the primary factor controlling porosity loss since the early diagenetic phases. In contrast, the early precipitation of pore-filling carbonate cement is assumed to have partly reduced the effect of compaction in quartzolithic and arkosic sandstones. Nonetheless, variations in petrophysical features among petrofacies can be traced back to the detrital composition, with the highest but less effective porosity associated with the presence of ductile grains, siliciclastic matrix and feldspars. Accordingly, this study reveals how linking detrital modes to the key petrophysical parameters may help obtain significant information about the regional control on the evolution of porosity and permeability in clastic turbiditic wedges of foreland domain settings as a baseline useful to further local and detailed studies.

On the structure and evolution of the Sorbas basin, S.E. Spain

Geological mapping and gravity surveying were used to constrain the shape of the Serravallian and Tortonian (U. Miocene) rocks of the Sorbas basin, SE Spain, and hence its tectonic evolution and place in the overall regional tectonic scheme. The present-day basin floor forms a westerly-deepening trough ∼2.0km deep, infilled with mainly clastic turbidites and mass-flow deposits. Basin-fill rocks underwent broadly ENE-WSW extension during accumulation, with extensional faulting on the flank of the Sierra Cabrera to the east. Acoustic velocity measurements demonstrate a petrophysical impact of the syndepositional deformation. Post-deposition but prior to uplift and erosion, the sediments were folded into a large-scale monoclinal structure, inferred to have formed in response to gravitational sliding of sediments off the rising western flank of the Sierra Cabrera. This fold structure was deformed and rotated by 3km of extension-related uplift of the basement block to the south during late Tortonian time, then erosion and flooding to produce a shallow-marine Messinian sequence, partially burying the older rocks. The geodynamic setting was the back-arc sector (the Betic zone) produced by Serravallian-Tortonian rollback of the Gibraltar arc subduction zone. It is bounded to the south by the Carboneras stretching transform fault, that accumulated about 40km of left-lateral offset coevally with sediment deposition and deformation within the Sorbas basin. These events are inferred to be geodynamically linked.

Burial and exhumation history of the Jameson Land Basin, East Greenland, estimated from thermochronological data from the Blokelv-1 core

Apatite fission-track analysis (AFTA) data in two Upper Jurassic core samples from the 231 m deep Blokelv-1 borehole, Jameson Land, East Greenland, combined with vitrinite reflectance data and regional AFTA data, define three palaeo-thermal episodes. We interpret localised early Eocene (55– 50 Ma) palaeotemperatures as representing localised early Eocene heating related to intrusive activity whereas we interpret late Eocene (40–35 Ma) and late Miocene (c. 10 Ma) palaeotemperatures as representing deeper burial followed by successive episodes of exhumation. For a palaeogeothermal gradient of 30°C/km and likely palaeo-surface temperatures, the late Eocene palaeotemperatures require that the Upper Jurassic marine section in the borehole was buried below a 2750 m thick cover of Upper Jurassic – Eocene rocks prior to the onset of late Eocene exhumation. As these sediments are now near outcrop at c. 200 m above sea level, they have been uplifted by at least 3 km since maximum burial during post-rift thermal subsidence. The results are consistent with estimates of rock uplift on Milne Land since the late Eocene and with interpretation of Ocean Drilling Program (ODP) data off South-East Greenland suggesting that mid-Cenozoic uplift of the margin triggered the marked influx of coarse clastic turbidites during the late Oligocene above a middle Eocene to upper Oligocene hiatus.

Virtual time-lapse seismic monitoring using fully coupled flow and geomechanical simulations

SEAM Time Lapse, a collaborative technical project of SEG and the Society of Petroleum Engineers, created integrated geologic, reservoir, and geophysical models to simulate temporal changes in the geometry and physical properties of a complex reservoir with the detail and accuracy needed to explain the subtle effects seen in time-lapse surveys of real oil fields. The geologic model consisted of 2 billion grid cells, representing a region 12.5 × 12.5 km in horizontal extent and 5 km in depth and including a 420 m thick reservoir with upper and lower units separated by an impermeable shale layer and offset by faults. Deepwater clastic turbidite channels and lobes were used to create a typical shallow Gulf of Mexico reservoir that also can serve as an analogue of other turbidite fields around the world. Stratigraphic detail within the reservoir was retained in the simulation model through careful finite-element meshing. The reservoir simulation computed the fully coupled three-phase fluid flow and linear geomechanical response in a production scenario involving 11 production wells and six water-injection wells penetrating the reservoir's three compartments. Seismic surveys were simulated with isotropic elastic-wave modeling before the start of production and after 27.5 months of production at a simulated rate of about 67,500 barrels per day, with about 32,500 barrels per day of water injection. Rock properties were updated by petrophysical models calibrated to turbidite systems in the Gulf of Mexico. Analysis of the model and simulations improves understanding of the complex interaction of fluid effects, pressure changes, and rock deformation. For example, compaction in the reservoir may cancel time-lapse fluid effects, and compaction or dilation in the surrounding shales may override the observed reservoir signal in the seismic bandwidth. Strain-induced velocity changes in the shales have a much larger effect on estimated time-lapse time shifts than do strain-induced changes in path lengths. Geomechanical effects impact the interpretation of 4D seismic difference attributes and require careful consideration. The integrated, full-physics SEAM approach used for this model provides a unique data set to explore these complex production effects and the value of 4D seismic surveys. The models and data, which also include simulations of time-lapse gravity and electromagnetic surveys, are publicly available through SEAM.

Dating the Gaofan and Hutuo Groups – Targets to investigate the Paleoproterozoic Great Oxidation Event in North China

There are several sedimentary units in North China that are proposed to be associated with the Paleoproterozoic Great Oxidation Event (GOE) and/or subsequent events; however, few of them have been precisely dated. In this study, deposition age of the greenschist facies Gaofan and Hutuo Groups is determined. Zircon grains liberated from a tuff layer (metamorphosed to sericite-quartz schist) in the upper part of the Mohe Formation (the second of the three formations of the Gaofan Group) yield a weighted average 207Pb/206Pb age of 2186±8Ma (n=7, MSWD=1.3), representing time of deposition. This age and the detrital zircon U-Pb ages of the basal feldspar quartzite (meta-siltstone), as well as the initial deposition age of the unconformably overlying Hutuo Group, confine the deposition age of the Gaofan Group to 2350–2150Ma. This result negates the Gaofan Group as one subgroup of the 2560–2510Ma Wutai greenstone belt. Zircons from the Banlaoyao mafic sill (meta-diabase) that intruded the Dongye Subgroup of the Hutuo Group yield an upper intercept U-Pb age of 2057±25Ma (n=14, MSWD=1.3), representing time of crystallization. Considering the age of the basalt in the first formation of the Doucun Subgroup and the tuff in the first formation of the Dongye Subgroup, the deposition age of the Doucun and Dongye Subgroups of the Hutuo Group is confined to 2150–2090Ma and 2090–2060Ma, respectively. These age brackets, as well as the available carbon and nitrogen isotope data indicate that the Zhangxianbu Formation of the Gaofan Group possibly recorded the GOE; whereas the Mohe-Yaokouqian Formations of the Gaofan Group and the Doucun-Dongye Subgroups of the Hutuo Group recorded the subsequent Lomagundi-Jatuli Event (LJE). However, the Lomagundi-Jatuli carbon excursions are hardly distinguishable from the Gaofan Group and the Doucun Subgroup (Hutuo Group) as both units consist of little inorganic carbon but terrestrial clastic turbidites.

Basin Analysis of the Albian–Santonian Xigaze Forearc, Lazi Region, South-Central Tibet

Abstract The Xigaze forearc basin records the evolution of the southern Lhasa terrane convergent margin, largely affected by Neo-Tethyan subduction processes, prior to the Paleocene Tethyan Himalaya–Eurasia collision. New geologic mapping and U-Pb detrital-zircon geochronologic data from the Lazi region, 340 km southeast of Lhasa, show that forearc basin sedimentation began ca. 110 Ma conformably atop the Yarlung–Tsangpo ophiolitic melange. By this time, the arc–trench system along the southern margin of the Lhasa terrane (Eurasia) consisted of an accretionary complex, overlying ophiolitic melange, the Xigaze forearc basin, and the Gangdese magmatic arc. There is no geological evidence in the Lazi region for more than one subduction zone between the southern Lhasa terrane margin and India. Sedimentological facies analysis from Albian to Santonian clastic and carbonate sedimentary rocks preserved in the Xigaze forearc basin indicate deep-marine sedimentation characterized by hemipelagic carbonate and volcanogenic sediment-gravity-flow deposits. Sandstone modal petrographic and U-Pb detrital-zircon geochronologic data reveal Asian continental margin, Gangdese magmatic arc, and central to northern Lhasa terrane provenance. During this time, basin fill was deposited in cycles of high and low sediment flux characterized by alternating successions of clastic turbidite inner- to outer-fan deposits and hemipelagic limestone and marlstone sequences. Along-strike differences in the timing of initial forearc basin sedimentation are likely the result of intra-basin topography and/or diachronous development of ophiolitic forearc basement.

Devonian ore clastic turbidites of the Molodezhnoe massive copper sulfide deposit, Southern Urals

We present data on the composition of Devonian ore clastic sediments accumulated at the boundaries of the orebodies of the Molodezhnoe massive copper sulfide deposit, Southern Urals. They are interpreted to be accumulated during the synsedimentary and postsedimentary stages. The synsedimentary stage was marked by gravity transport and accumulation of products of the destruction of high-temperature black smoker chimneys. During the postsedimentary stage, the deposits underwent two types of mineralogical and geochemical transformation: (1) infiltration-metasomatic effects of ore-bearing solutions and infiltration effects of seawater causing redistribution of copper and carbonates in the sediments and formation of authigenic minerals (chalcopyrite, siderite, secondary calcite, and apatite and replacement of aluminosilicates by chlorite) and (2) dehydration accompanied by the replacement of iron hydroxides by hematite and amorphous silica by quartz. Temperatures of the postsedimentary alteration were estimated from a fluid inclusion study and thermodynamic computer modeling as 150–250°C and correspond to the metagenesis stage (burial metamorphism). The organic matter composition was characterized by TOC, hydrogen and oxygen indices (HI and OI), and δ13C values.

High amplitude redox changes in the late Early Triassic of South China and the Smithian–Spathian extinction

The Early Triassic was a time of remarkably high temperatures, large carbon cycle perturbations and episodes of widespread ocean anoxia. The sediments in the Nanpanjiang Basin of South China provide superb opportunities to examine the sedimentary response to these extreme conditions especially during the crisis interval at the Smithian-Spathian (S-S) boundary. We have investigated a deep water section at Jiarong and a shallower water section at Mingtang. These contain a range of including black shales, micritic limestone units and rudaceous carbonate event beds that include flat pebble conglomerates and breccia debrites that bear similarities to the hybrid event beds seen in clastic turbidite successions.Redox proxies (pyrite framboids and trace metals) reveal that widespread anoxia in the late Smithian persisted into the Novispathodus pingdingshanensis Zone of the early Spathian before a sharp transition to highly oxygenated griotte facies (red marine strata) in the Icriospathodus collinsoni Zone that records an oxic rebound. Benthic faunas are locally common but of low diversity and dominated by thin-shelled bivalves and ostracodes with small foraminifers and exceptionally rare fish remains. Bioturbation was intense only in the early-middle Spathian (I. collinsoni conodont zone) Griotte facies. Anoxia and extremely high temperatures probably played a role in severely restricting the abundance of fish and the small sizes of marine invertebrates at this time. The presence of ooids and seafloor fan cements in our study sections indicates highly saturated conditions rather than acidification at the end of the Smithian.

The Altai-Mongolia terrane in the Central Asian Orogenic Belt (CAOB): A peri-Gondwana one? Evidence from zircon U–Pb, Hf isotopes and REE abundance

Understanding the tectonic nature of the Altai-Mongolia terrane is significant in tectonic reconstruction of the Central Asian Orogenic Belt (CAOB), a Paleozoic accretionary one similar to the present-day SW Pacific archipelago. Uncertainty in stratigraphic frame of this terrane has led to many controversies in tectonic interpretation. New zircon U–Pb ages, along with previously published geological data help us to build a simplified stratigraphic frame, consisting of (1) the Middle to Upper Cambrian clastic turbidite intercalated with bimodal volcanics, and (2) the Devonian arc-like volcanics and shallow marine sediments. New geochemical data for the meta-basalts suggests that the bimodal volcanics are rift-affinity. Detrital or xenocrytic zircon U–Pb data, combined with zircon ɛ Hf( t) results reveals that the Altai-Mongolia terrane has a similar Neoproterozoic to Early Paleozoic tectonothermal history in comparison with the Cadomian-Avalonian orogenic belt. This similarity suggests that the Altai-Mongolia terrane likely is a peri-Gondwana one separated from the northern margin (at present geographic coordinate) of Gondwana during the Middle to Late Cambrian. The rifting might have led to the generation of a branch of the Paleoasian Ocean, an eastern equivalence of the Rheic Ocean. The Silurian to Devonian northward subduction (at present geographic coordinate) of the Paleoasian oceanic crust has given rise to an Andean type arc along the southern margin of the Altai-Mongolia terrane. New magmatic zircon U–Pb ages combined with zircon ɛ Hf( t) and REE results suggest that the Silurian to Devonian arc-like magma were generally derived from the depleted mantle. Our findings shed some light on tectonic reconstruction of central Eurasia and allow its comparison with some pre-Variscan massifs in Europe.

Compound seismic modelling of the Ainsa II turbidite system, Spain: Application to deep-water channel systems offshore Angola

Seismic models of outcrops are essential for qualifying petroleum targets, because they bridge a critical gap in both resolution and scale between architectural geometries observed in outcrops and in seismic data. The paper describes how a seismic model of the deep-marine Ainsa II turbidite system of the St. Vicente Formation in the Southern Pyrenees, Spain is developed and used as an analogue for exploration prospects in the Quifangondo Formation, Lower Congo Basin offshore Angola. First, geometry and lithology information from the Ainsa II outcrop were utilised to construct a detailed earth model. The Ainsa II turbidite system consists of at least five clastic turbidite units bounded by erosional surfaces, where only the youngest unit is steep-sided. The input geo-model (earth model) includes stratigraphic information, such as truncations, onlaps, thickening/thinning of beds, condensed sections and detailed lateral changes in lithology. Second, subsurface petrophysical properties (velocity and density logs) necessary to build the forward seismic models were extracted from an offshore well in Angola. This allowed us to utilise the outcrop as a seismic analogue for a relatively narrow, laterally aggrading turbidite system offshore Angola. The seismic models showed that it is difficult to image and identify amalgamated beds inside a larger unit, particularly if beds have predominantly the same facies associations and weakly dipping basal boundaries (as in the lower part of the Ainsa II system). Second, seismic modelling helped to highlight areas where it is difficult to distinguish between seismic onlap and true stratigraphic onlap. Information which decides when evaluating stratigraphic traps. We also found that lateral degradation in amplitude may represent a lateral change in lithology (e.g. from sandy channel deposits to levee-overbank heterolithics). Integration of outcrop mapping, well data, 3D seismic interpretation and forward seismic modelling, has enabled us to study deep-water depositional elements at a sub-seismic scale. This has resulted in enhanced prediction of the lithology distribution and architecture of the turbidite system offshore Angola, and gives new insights into the 3D geometry of the Ainsa II system.

Late Triassic radiolarians of southern Cyprus

Radiolarians from the Upper Triassic of the allochthonous Mamonia Assemblage of southern Cyprus are considered. The Phasoula Formation, composed of basic volcanics, with lenses and interbeds of micritic limestones and cherts, contains (1) a Lower Norian assemblage with Capnodoce crystallina-Trialatus robustus, which also includes Capnodoce anapetes De Wever, Capnuchosphaera deweveri Kozur et Mostler, C. theloides De Wever, Deflandrecyrtium curvatum Kozur et Mostler, Icrioma cruciformis Tekin, Kahlerosphaera norica Kozur et Mock, Kinyrosphaera helicata Bragin, Mostlericyrtium sitepesiformis Tekin, Palaeosaturnalis latiannulatus Kozur et Mostler, Spongostylus tortilis Kozur et Mostler, Xiphotheca rugosa Bragin, and Zhamojdasphaera proceruspinosa Lahm; (2) a Middle Norian assemblage with Capnodoce sarisa accompanied by Loffa mulleri Pessagno, Nabolella trispinosa Bragin, and Praexehasaturnalis tenuispinosus (Donofrio et Mostler); and (3) an Upper Norian assemblage with Livarella densiporata-Lysemelas olbia accompanied by Pentactinocarpus sevaticus Kozur et Mostler, Praemesosaturnalis multidentatus (Kozur et Mostler), and others. This assemblage also occurs in clastic turbidites of the Vlambouros Formation. In the sections of southern Cyprus, radiolarian zones are recognized that correspond to the zones previously established in the Far East of Russia, which include Capnodoce crystallina (Lower and Middle Norian) and Lysemelas olbia (lower part of the Upper Norian). Radiolarians belonging to three orders, 24 families, 59 genera, and 101 species are described; of them 2 genera, 9 species, and 1 subspecies were previously described by the author; 14 new species and 1 new subspecies are established. The diagnoses of many genera and species are emended, the stratigraphic and geographical ranges of the majority of taxa are substantially expanded.

Uppermost Cretaceous–Lower Tertiary Ulukışla Basin, south-central Turkey: sedimentary evolution of part of a unified basin complex within an evolving Neotethyan suture zone

The Ulukışla Basin, the southerly and best exposed of the Lower Tertiary Central Anatolian Basins, sheds light on one of the outstanding problems of the tectonic assembly of suture zones: how large deep-water basins can form within a zone of regional plate convergence. The oldest Ulukışla Basin sediments, of Maastrichtian age, transgressively overlie mélange and ophiolitic rocks that were emplaced southwards onto the Tauride microcontinent during the latest Cretaceous time. The Niğde-Kirşehir Massif forming the northern basin margin probably represents another rifted continental fragment that was surrounded by oceanic crust during Mesozoic time. The stratigraphic succession of the Ulukışla Basin begins with the deposition of shallow-marine carbonates of Maastrichtian–Early Palaeocene age, then passes upwards into slope-facies carbonates, with localised sedimentary breccias and channelised units, followed by deep-water clastic turbidites of Middle Palaeocene–Early Eocene age. This was followed by the extrusion of c. 2000 m of basic volcanic rocks during Early to Mid Eocene time. After volcanism ended, coral-bearing neritic carbonates and nummulitic shelf sediments accumulated along the northern and southern margins of the basin, respectively. Deposition of the Ulukışla Basin ended with gypsum deposits including turbidites, debris flows, and sabkhas, followed by a regional Oligocene unconformity. The Ulukışla Basin is interpreted as the result of extension (or transtension) coupled with subsidence and basic volcanism. After post-volcanic subsidence, the basin was terminated by regional convergence, culminating in thrusting and folding in Late Eocene time. Comparisons of the Ulukışla Basin with the adjacent central Anatolian basins (e.g. Tuzgölü, Sivas and Şarkişla) support the view that these basins formed parts of a regional transtensional (to extensional) basin system. In our preferred hypothesis, the Ulukışla Basin developed during an intermediate stage of continental collision, after steady-state subduction of oceanic crust had more or less ended (“soft collision”), but before the opposing Tauride and Eurasian continental units forcefully collided (“hard collision”). Late Eocene forceful collision terminated the basinal evolution and initiated uplift of the Taurus Mountains.

Magnetic Stratigraphy from Deep Clastic Turbidites: An Example from the Eocene Hecho Group (Southern Pyrenees)

A paleomagnetic study of a 2200-meter thick section of clastic turbidites from the Eocene Hecho group (southcentral Pyrenees, Spain) allows defining its magnetostratigraphic record. Natural remanent magnetization is carried by up to three components: a viscous low-temperature component; a second component unblocked between 300°C and 345°C, likely carried by iron-sulphides; and a third component which demagnetizes at temperatures higher than 345°C and is likely carried by magnetite. The second and third components may display opposite polarities at the same site. The “magnetite” component delineates different polarity zones and has a more consistent behavior along section if compared with the “iron-sulphide” component, which displays either a normal or a reverse direction without any stratigraphic consistency along section. The bulk of iron sulphides are interpreted to be secondary in origin and to carry diagenetic overprints acquired at different times after deposition and the “magnetite” component is taken as the characteristic primary magnetization. As supported by biostratigraphy, the section is correlated from chrons C20r to C18n.2n (Lutetian-Bartonian transition), which indicates a mean sediment accumulation rate of about 52 cm/ky for the studied section. The new chronostratigraphy allows constraining the age of the upper Hecho Group (Banaston and Jaca allogroups) to an unprecedented level and is consistent with previous magnetostratigraphic work in younger sediments from the Jaca Basin. Deep clastic sedimentary systems should not be neglected as a target for magnetostratigraphic studies despite diagenetic growth of secondary minerals may mask the primary signal.

3D Modelling/Visualization Guides Horizontal Well Program In Wilmington Field

Abstract Computerized mapping, modelling, and simulation programs used in conjunction with advanced geosteering technology have helped to successfully tap bypassed, heavy oil in California's mature Wilmington oil field. While THUMS (Long Beach Company) and Tidelands Oil Production Company-the two field contractors working separate portions of the field operated by the City of Long Beach-have taken different approaches to field optimization using horizontal drilling, each has been rewarded by significant additional production. The following text specifically focusses on how 3D visualization tools have improved Tidelands Oil Production's recovery factor in the "Old Wilmington" or western portion of the field. Data from THUMS' Long Beach Unit, the eastern portion of the field, are included where needed to provide a thorough field overview. The Challenge The Wilmington oil field produces from Pliocene and Miocene clastic slope and basin turbidite sandstones. The individual reservoirs are defined by graded sequences of sandstone that are interlayered with siltstones and shales. The entire sequence is folded and faulted. Even the typically rhythmically deposited sequences have lenticular lobate shapes and are complicated by basal scour, onlapping, and channelling. The result is a sequence of rocks that often appears to be uniform but is not. These complexities also result in permeability variations that reduce the producibility of the sandstones, impact waterflooding, and result in a substantial amount of bypassed oil. Wilmington Field's stratigraphic layers are divided into seven producing zones, 52 subzones, and locally into even finer subzones(1). The finer subdivisions are defined as hydrologic bodies or depositional sequences. Many techniques are applied todefine the thinner sand bodies into unique units, including core description combined with log-rock typing, detailed log correlation, production/injection history matching, bypassed pay saturation analysis on recent pass-through wells and reservoir simulation(2). There is still much to be learned about the intricacies of Wilmington Field's reservoirs, although today's computerized visualization tools coupled with advanced while-drilling measurements have significantly contributed to the collective knowledgebase about this field. Meanwhile, its poorly drained sands remain ideal targets for horizontal drilling. History in the Long Beach Unit The eastern portion of Wilmington Field was originally produced with more than 1,000 wells drilled between 1965 and 1982. Even given the very long completion intervals used and water injection for pressure support, oil remained in pockets of tight, thin sands, as well in areas with poor injection support. Widely ranging permeabilities and faulting caused typically viscous oil (12.5 ° to 16 °API) to be left. behind in 20 to 50 ft. (6 to 15 m) thick sand units. An additional 460 wells were drilled in the Long Beach Unit from 1982 to 1986 using a "sub-zone" approach to improve sweep efficiency and another 160 million barrels (bbl) or 25.4 million cubic meters (m3) were produced. Then, after 1986 bypassed sands were selectively perforated in even finer intervals.

Cretaceous Subsurface Geology of the Middle East Region

ABSTRACT A regional structure contour map at Near Top Cretaceous is based on hundreds of well tops from extensive bibliographic references from throughout the Middle East. This structure map shows strong basin asymmetry. Major faults are shown in outcrop and/or suspected at basement or intermediate levels, based in part on gravity and magnetics modeling, published and in ‘open file’ studies. Major uplifts associated with several super-giant oil and gas fields are clearly indicated even at the shallow Cretaceous level (Ghawar Anticline, Qatar Arch, Burgan-Khurais Trend, etc.), even at a very regional scale with a contour interval of 1,000 feet. Isopach maps of Upper, Middle, and Lower Cretaceous are contoured at intervals of 500 feet. Each of these three isopach maps is overprinted in color to show generalized lithofacies trends. Lower and Middle Cretaceous deltaic sandstone fairways on the western shelf provide excellent reservoir rocks for a trend containing many of the world’s very largest oil fields. Somewhat more basinward, predominantly carbonate facies include oil reservoirs in the Upper, Middle, and Lower Cretaceous. Deepest facies lie beneath the Zagros Foothills Belt in coastal Iran and eastern Iraq. This is particularly true for the Upper Cretaceous, where Coniacian to Maestrichtian thick, deep basinal shales, cherts, clastic turbidites, and slumped exotic blocks of ophiolites mark the northeastern border of the Late Cretaceous basin as it approaches the Main Zagros Fault and an assumed subduction zone underthrusting the Iranian Plate or Eurasia.

北海道の古第三系堆積盆の変遷

The Paleogene of Hokkaido is dominated in coal-bearing formations. Four stages are discriminated in evolution of the Paleogene sedimentary basins, above all, coal basins. A subsiding deep trough extended in the axial part of Hokkaido and southeastern Sakhalin between the East Asian continent and the Okhotsk microcontinent, and was filled with a 5 km-thick, clastic turbidite facies in the Paleocene. At the stage I in the latest Paleocene to earliest Eocene, the Haboro coal basin formed as a fore-arc basin in north-central Hokkaido on the west side of the trough. By contrast, the Nemuro shelfal clastic basin existed on the east side of the trough and encircled the southwest margin of the microcontinent. A drastic paleogeographic change occurred in the Early Eocene. The axial part of Hokkaido, which had been occupied by the deep trough, turned to upheaving associated with the Hidaka metamorphism, probably due to collision of the Okhotsk microcontinent against the East Asian continent. As a result, the trough and the sedimentary basins on its both sides were converted into a land. At the stage II in the middle Middle Eocene, the Ishikari-Uryu coal basin formed as an inter-arc basin in south-central Hokkaido where the Late Cretaceous continental fore-arc basin preexisted. The coal basin was first restricted to the Ishikari coalfield in the south, and later it enlarged to the Uryu coalfield in the north by the northward Wakanabe-Shiraki transgression. At the stage III in the late Middle Eocene, the Ishikari-Uryu coal basin extended to the west due to the subsidence of the Kabato-Rebun Cretaceous volcanic terrane. Basal fanglomerates, a few hundreds meters thick, overlapped the Cretaceous basement rocks in the west part of the coal basin. At the same stage, the Kushiro coal basin of the Kushiro coalfield formed incisely within the Nemuro shelfal basin in southeast Hokkaido, although it extended to the west and overlapped the Cretaceous basement rocks of the Tokoro terrane with conspicuous fanglomerates. At the stage IV in the Late Eocene to Early Oligocene, all coal basins of the previous stages subsided to the site of the deposition of shallow-marine to upper bathyal muddy sediments. In central Hokkaido, the Late Eocene marine basin of the Poronai Formation and its equivalents extended northwards from the Ishikari-Uryu coal basin to the Haboro coal basin of the stage I, and further to the north, to Sakhalin. However, the Early Oligocene mudrocks of the Momijiyama Formation was recognized only in the southern part of the Ishikari coalfield, and showed a restricted embayment environment. In east Hokkaido, the Early Oligocene continental sediments of the Wakamatsuzawa Formation overlapped the Cretaceous basement rocks of the Tokoro terrane on the northwest of the Late Eocene to Early Oligocene marine basin of the Onbetsu Group.The Paleogene of Hokkaido has a total thickness of two to five kilometers. The Paleogene strata of central Hokkaido are commonly extensively folded and overthrusted. The thick and folded Paleogene in central Hokkaido is undoubtedly traceable to Sakhalin, and possibly to the west side of central and northern Kamchatska.The third-ordered eustatic curves of Haq et al. (1987) indicate that, as a whole, Late Eocene is low stand while Early Oligocene is high stand. In Hokkaido, however, marine basins are most widespread in the Late Eocene, whereas they are restricted in the Early Oligocene. Local tectonic movements are therefore essential for the evolution of the Paleogene sedimentary basins in Hokkaido.

Diachronous uplift and recycling of sedimentary basins during Cenozoic tectonic transpression, northeastern Caribbean plate margin

Four marine sedimentary sequences of Late Cretaceous to Pleistocene age crop out in the 320 km long, 1 to 30 km wide Peralta-Rio Ocoa belt of Hispaniola (Haiti and Dominican Republic). The four sedimentary sequences are dominated by marine turbiditic rocks ranging up to 11.5 km in apparent thickness and exhibit mainly northwest to southeast, belt-parallel paleocurrents. The three younger sequences are generally in fault or unconformable contact with the underlying sequence. From northwest to southeast, the four belts become progressively wider, younger, less metamorphosed, less folded and faulted, and lower in topographic elevation. We interpret the three younger sequences as a syn-tectonic stratigraphic record of diachronous, northwest to southeast transpressional closure of a Coniacian-Danian back-arc basin represented by the oldest exposed sedimentary sequence at the northwest end of the belt and inferred at depth beneath the three younger sequences. Using the stratigraphic record, we infer the following three stages in the closure of the back-arc basin and the overlying younger basins. Stage One: latest Cretaceous to early Late Eocene closure. A minimum of 11 km of Paleocene turbidites and limestone in the Padre las Casas area and a minimum of 11.5 km of Early Eocene to early Late Eocene pelagic limestone, mudstone, sandstone, and siltstone in the Peralta and Sierra El Numero areas was deposited in an elongate basin derived from the first stage of latest Cretaceous-early Late Eocene closure and erosion of the Coniacian-Danian back-arc basin to the northwest. Paleocene-Eocene turbidites in both areas contain large amounts of reworked Campanian-Paleocene microfauna and exhibit northwest to southeast belt-parallel paleocurrents. Syn-deformational features in Eocene sedimentary rocks in the Peralta and Sierra El Numero area indicate that convergent deformation accompanied sedimentation (Witschard and Dolan, 1990). Stage Two: Middle Eocene to Early Miocene closure. Up to 8.6 km of Middle Eocene to Early Miocene turbidites, debris flows, and olistostromes of the Rio Ocoa Group of the Dominican Republic were deposited in an elongate basin derived from Middle Eocene to Early Miocene closure, uplift, and erosion of the Peralta Group to the west and northwest. Turbidites contain reworked Eocene faunas and lithologies similar to those in the underlying Peralta Group and exhibit northwest to southeast, belt-parallel paleocurrents. Erosion of Late Cretaceous island arc and overlying Paleogene carbonate rocks of the Cordillera Central provided additional belt-perpendicular sources of coarser-grained conglomerate and olistostromes. Following deposition of the Rio Ocoa Group, both the Peralta and the Rio Ocoa Group were deformed in a southwest-verging fold-and-thrust belt during the Early Miocene. Stage Three: Middle Miocene to Recent closure. Up to 1.5 km of Middle Miocene to Pleistocene sandstone, conglomerate, and reefal limestone of the Ingenio Caei Group of the Dominican Republic were deposited in an elongate basin above a pronounced angular unconformity developed on the older, deformed rocks of the Rio Ocoa Group. Sediments of the Ingenio Caei Group were derived from Middle Miocene to Recent closure, uplift, and erosion of the Rio Ocoa and Peralta Groups to the northwest. Onshore exposures of the Ingenio Caei Group can be correlated with submerged strata seen on seismic reflection profiles across the offshore San Pedro basin. Recent submarine clastic sedimentation in the San Pedro basin is dominated by northwest to southeast influx of clastic turbidites derived from active erosion of onshore exposures of the Rio Ocoa and Ingenio Caei Groups. Middle Miocene (?) to Recent sediments of the San Pedro basin are being actively shortened by northward to northeastward-directed underthrusting along the Muertos trench. Underthrusting at the Muertos trench is suggested to represent ongoing closure of the back-arc basin thought to be deeply buried beneath the San Pedro basin. Tectonic transpression along the closing back-arc basin marks one edge of an actively deforming microplate within the North America-Caribbean strike-slip plate boundary zone.

Geology and depositional history of Cenozoic sediments of the Bengal Basin of Bangladesh

The Bengal Basin lies on the eastern side of the Indian subcontinent and occupies the northern part of the Bengal geosyncline. Sedimentation started with the breakup of Gondwanaland. Cretaceous fluvial and deltaic sedimentation was slowly taken over by open marine and deep-sea fan sedimentation. By the Mid Eocene, the basin was witnessing the maximum marine transgression and most of the stable shelf was in carbonate regime, as part of the extended Tethyan carbonate shelf. The deeper part of the basin was dominated by deep-sea fan sedimentation fed by clastic turbidites from the northeast with limited carbonate turbidites from the shelf. The collision of the Indian plate with the Tibetan plate and with the Burmese plate in the Miocene resulted in a rapid switch in sedimentation pattern in the Bengal Basin. The basin structure and sedimentation were both strongly influenced by the collision pattern of the plates and by the uplift of the Himalayas. With the Miocene collision of the Indian Plate, there was a change from flysch to molasse sedimentation. The present-day basin configuration with the Ganges-Bramaputra delta on the north and the Bengal deep sea fan on the south was established in the Pliocene. The stratigraphic and tectonic framework of the Bengal Basin suggest that the deposition of hydrocarbon source rocks was associated with the flysch sedimentation, whereas formation of traps and deposition of most of the reservoir rocks in the basin was associated with Mid-Pliocene tectonic movements and with the molasse sedimentation. The pre-Mid-Miocene sediments now deeply buried in the deeper part of the Bengal Basin and in the eastern folded belt are the major source of gas of the eastern folded belt of the basin.

Topographical and geological maps of Hall Land, North Greenland. Description of a computer- supported photogrammetrical research programme for production of new maps, and the Lower Palaeozoic and surficial geology

Topographical and geological map sheets covering the northern part of Hall Land (81-82°N) are presented – an area of about 3000 km2. The maps are the products of a research programme in which newly developed photogrammetric techniques have been used in the interpretation and compilation of the topography and the geology (both solid and surficial). The topographical map has been constructed with a minimum of geodetic ground control. The topographic contours have been calculated from a digital elevation model using computer programmes, and automatically plotted out. The geological map has been hand-drawn from 74 manuscript sheets compiled from aerial photograph models on second-order analog stereo-plotting instruments with computer facilities. The maps, the photogrammetric programme and the solid and surficial geology are described in seven chapters. The first two provide an introductory background that explains the motivation for the research, summarises the history of cartographic, geodetic and geologic work and provides a status of research at the start of the programme. The third chapter discusses the various aspects of the photogrammetric programme, instrumentation and the on-line computer facilities utilised, and is followed by a chapter dealing with compilation method, map presentation and assessment of cartographic accuracy compared to previous maps and modern geodetic ground data. The next chapter describes the topography and geomorphology and relates the three main physiographic provinces to the solid and surficial geology. The penultimate chapter outlines the stratigraphy and structure of the Upper Ordovician-Silurian (Llandovery-Pridoli) section through the E-W trending Franklinian basin. In Ordovician-earliest Silurian time, the map area was part of the carbonate platform; in the Llandovery a major shift southwards of the deep-water basin occurred. The Silurian succession displays a regional facies change from platform carbonates in the south, through a major reef belt on the shelf and upper slope to, in the north, clastic turbidites of the lower slope and trough. Facies transitions and interdigitation of shelf-slope-trough lithologies are complex. The northern part of the map exposes the autochthonous margin of the mid-Palaeozoic North Greenland fold belt characterised by E-W folds. The regional structure is an asymmetric synclinorium; a decollement zone probably occurs in the shale sequence that overlies the Lower Silurian carbonate platform. The final chapter describes eight groups of Quaternary deposits and features: moraine, fluviatile-glaciofluvial, marine, lacustrine, colluvial, solifluction, aeolian and periglacial. Hall Land was formerly entirely ice covered, and deposits of several ice advances are preserved; six major marginal moraine systems are defined. Marine deposits are prominent and terrace levels and raised shorelines are well preserved; the Holocene marine limit is at least 125 m above present sea level. Major events are placed within a Pleistocene-Holocene chronostratigraphic framework. Comments on place names are given in an appendix.