Abstract Reservoir production is highly dependent on reservoir models. A key problem faced in the development of a hydrocarbon reservoir is that of constructing a reservoir model that can generate reliable production forecasts under various development scenarios. Therefore, geological models have to be built in three dimensions (3D). Unfortunately, manual construction of 3D geological models (deterministically) is almost impossible, which explains why geologists often limit their interpretation to two dimensional (2D) correlation panels, fence-diagrams or maps. Consequently, geological conceptual models are rarely included or considerably simplified in reservoir models used for flow simulations and replaced by stochastic or geostatistic approaches. In spite of this admission of failure, sedimentological cross-sections and maps contain most of the knowledge and concepts of sedimentologists. They represent the outcome of sedimentological studies, including available well data, seismic interpretation and especially sedimentological and environmental concepts, incorporating all facies transitions and successions in a high-resolution stratigraphic framework. They allow fine temporal- and spatial-scale sedimentological heterogeneities to be identified. The integration of these fine-scale sedimentological heterogeneities is an essential step in improving the precision and accuracy of static reservoir models and volumetric calculations. This paper demonstrates the quantitative influence of introducing sedimentological information into the reservoir characterization workflow using a simple deterministic workflow. The described incorporation of sedimentological knowledge through facies 3D proportions cubes allows a direct assessment to facies distribution multi-realization scheme and associated uncertainties by applying stochastic simulations.

Abstract Hardly detected with logs and recognized with difficulty on cores, clastic injectites (sills and dikes) can be troublemakers in oil-field development. Moreover, they provide a precious record of early fracturation. To predict their geometry, extension, and relationship to their feeders, field analysis of selected analog outcrops is conducted to propose some simple rules. In southeast France, the Aptian–Albian formation provides exceptional outcrops (Bevons, Rosans, and Nyons) where it is possible to characterize large sets of injectites: dikes and sills are associated in the same metric-to-kilometric network. The injection occurred per ascensum (more frequently) or per descensum, during or after the sand deposition. Specific geometric-based field methods have been developed to analyze the geometry based on the best conditions. A three-dimensional (3-D) model of the Rosans area injectite network has been built through gOcad ™ tool using outcrop analysis and an original, very high-resolution twodimensional seismic acquisition (0.6 km 2 ; 0.23 mi 2 ). This field analysis, the seismic survey, and the 3-D modeling provide some keys to consider possible occurrences of injectites and associated facies related to a turbiditic channel fill. We dedicate this article to the memory of our colleagues and friends G. P. Allen and D. Claude.

Abstract Conflicting views have recently been expressed about the deep structure of the Gulf of Mexico, as interpretations cover the whole spectrum from oceanic to extended continental crust. The interpretation proposed here favors the former interpretation, as evidence of the oceanic character is interpreted from both seismic (reflection and refraction) and magnetic datasets. The core of the interpretation is the recognition of a package of seaward-dipping reflectors ( SDRs ) below the Louann Salt at the base of the Florida escarpment. Gravity and magnetic data and seismic velocity analysis, indicate that this package of seismic events is dense, magnetic, and has high velocities. Therefore, they are unlikely to consist of Triassic siliciclastics. Consequently, it is interpreted that these events correspond to the earliest stages of subaerial accretion of volcanic crust during spreading, before the Gulf of Mexico was in communication with the world ocean. In the more distal part of the basin, the morphology of the basement on the abyssal plain is considered as representing “classic” oceanic crust having a ridge/transform morphology that is based on structural mapping of seismic data. Both the SDRs wedge and the structural trends observed on the oceanic crust of the abyssal plain indicate a spreading axis roughly trending southeast, indicating that they can be part of the same episode of volcanic/oceanic opening. The 50 km-wide intermediate area between the SDRs wedge and the oceanic crust in the abyssal plain, in the absence of good seismic imaging, is interpreted as an area emplaced by volcanic/oceanic spreading in the same episode. The Jurassic Louann salt overlies the SDRs wedge (emplaced in subaerial conditions) and therefore postdates the beginning of the volcanic spreading along the studied transect. It also overlies the intermediate area seaward: salt deposition is therefore interpreted to have ended after the emplacement of about 100 km of volcanic/oceanic crust. Consequently, volcanic/oceanic spreading in the eastern Gulf of Mexico started before salt was deposited along the studied part of the margin, continued during salt deposition, and ceased in the Early Cretaceous.

Abstract A companion paper ( Imbert, 2005 , this volume) interprets the abyssal plain of the eastern Gulf of Mexico as an oceanic domain bounded to the northeast by a volcanic margin associated with the initial opening of the basin prior to the end of Louann Salt deposition. This paper explores the consequences of this interpretation on the opening model, at the scale of the entire Gulf of Mexico. Magnetic anomaly maps are compatible with structural elements derived from seismic interpretations and are interpreted in the deep Gulf of Mexico domain as magnetic stripes recording the reversals of the magnetic field of the earth during the opening of the Gulf of Mexico. This interpretation allows us to closely define the successive stages of opening. This interpretation is tested against one potential major time line, the seaward limit of deposition of the Jurassic salt. This limit is observed in two domains; the Louann salt in the north and the Campeche and associated salt basins in the south. It is impossible to image the initial depositional limits of the salt due to the limits of seismic visibility. The proposed reconstruction implies a depositional limit of the salt some 100 km inward of the Sigsbee escarpment to avoid overlap. Modeling shows that syn-opening flowage of the salt after the end of evaporitic conditions, under the combined effect of gravity and thermal subsidence of the young oceanic crust, could lead to this order of magnitude of salt displacement. This would also explain why there appears to be more early extension than compression in several areas of the Gulf of Mexico: part of the compression has been absorbed by salt flowage on opening crust ( i.e. , intra-salt and cryptic). A general reconstruction of the early stages of opening of the Gulf of Mexico is proposed under these hypotheses.

Abstract The early years of process–response sedimentological research were strongly focused on deltas, for a variety of reasons. They were important for accumulation of oil, gas, and coal; they were environmentally sensitive; they were readily accessible to academic communities; and consequently they were intensely studied. This characterized the decades of the 1960s and 1970s. With the shift in emphasis towards predictive stratigraphy and the stacking of depositional systems, and away from the strati-graphic architecture of depositional systems themselves, research on deltas reached a plateau in the 1980s and early 1990s. Today, however, the widespread use of shallow- as well as deep-penetration seismic data, cores from subsurface reservoirs, vibracores from modern environments, sophisticated oceanographic tools, and numerical modeling has resulted in a rejuvenation in delta research. In addition, a global province that hitherto had received relatively little attention increasingly became a focus for research—the equatorial zone of Southeast Asia. It is the objective of this volume to bring to the fore a category of deltas with which many sedimentologists and stratigraphers are, at best, vaguely familiar. It is expected that this volume also will stimulate new research on tropical deltas by highlighting how their facies and stratigraphic architectures differ from mid- and high-latitude ones, by emphasizing their significance to the global sediment budget, and by stressing their uniqueness within a petroleum systems framework. This special publication emphasizes the need for models intrinsic to tropical deltas of Southeast Asia to supplement the more conventional general models currently in vogue, based on past studies

Abstract Deltas in the shallow marine epicontinental Gulf of Carpentaria have developed in the slowly subsiding Karumba Basin and are influenced by monsoonal fluvial discharge and a diurnal tidal regime. The McArthur delta is protected from wave action by offshore islands, and the restricted width of the delta is a function of bedrock outcrops near the delta mouth. The upper delta plain is characterized by fluvial lateral-accretion point-bar deposits. In the lower delta plain, progradation has resulted in a buildup of shelly delta-front sands overlain by muddy intertidal and supratidal deposits. The latter dry mudflats are areas of wind deflation and may equate to emergent surfaces in older analogues. Fluvially active distributary channels have a relatively uniform width whereas abandoned channels adopt a tapering tide-dominated form. This delta shows progressive influence of riverine processes downstream onto the lower delta plain. Although the tide-dominated channels have a high sinuosity, their patterns reflect former fluvial channels. The Gilbert River, by contrast, is not bedrock-controlled and contains a prograding coastal wedge of Holocene sediment extending laterally for a distance of 125 km along the coast. The subaerial portion of the delta has prograded 15–20 km during the past 6,500 years, and the subsurface facies show prodelta mud overlain successively by delta-front and subtidal sands, strandline beach and chenier ridges, and intertidal to supratidal mudflats. Thin floodplain deposits cover the inner portion of the delta. Changes in the locus of sedimentation result from fluvial avulsions and account for age variations in the local subtidal sand and beach-ridge accumulations. Deltas in the northern Australian region are not adequately defined by simple morphological classifications. They drain from geologically mature landscapes and illustrate complex morphological patterns that develop in response to specific tidal and fluvial regimes. No major incision would have occurred around the Gulf of Carpentaria during the last low stand of sea level because the onshore and offshore gradients are equivalent, and the Holocene deposits have built out as temporary wedges of sediment (up to 30 km wide and 10 m thick) adjacent to the present shoreline.

Abstract The Tseng-wen River is a small monsoon-regulated mountainous river on the wave-dominated west coast of Taiwan. Historical maps reveal a trend of shoreline accretion through delta progradation during the past two and a half centuries until the mid 1970s. Two decades after the completion of a reservoir in the upper reaches of the river in 1974, the depositional setting at the river mouth gradually changed from a progradational deltaic system to an estuarine system. Presently, during the flood season, most of the river-borne sediment is in the mud-size fraction. On the time scale of a tidal cycle, the area of initial river sediment discharge is restricted to the nearshore region in the immediate vicinity of the river mouth. Because of tidal movements in the Taiwan Strait, dispersal patterns of river-borne sediment are dominated by longshore tidal currents that display bilateral swings in the course of a tidal cycle. Yet, on a long-term basis the river discharge is the most important sediment source for the coast and nearshore region. One other important sediment source for the area is the reworked offshore palimpsest sediments. After initial deposition, wave agitation is the most important mechanism affecting sediment entrainment. Several lines of evidence, including grain-size distributions on the sea floor (on the scales of years), measurements of in situ flow and turbidity (on the scale of days), and numerical model simulations of the plume (on the scale of tidal cycles) indicate net northward (flood-oriented) sediment dispersal. The Tseng-wen River represents the type of river that has dwindling influence on the surrounding coast in terms of sediment supply and textural imprint.

Abstract Thick, domed peat deposits dominate most of the surface of the mesotidal to macrotidal Rajang River delta, tidally influenced alluvial valley, and adjacent coastal plain. Northeast-striking shoreline terrace sands that crop out along the landward margin of the delta and coastal plain and gravel outcrops in the alluvial valley are the surface expression of the VIIa highstand surface of 125 ka (oxygen isotope stage 5e). The upper few meters of the VIIa surface have undergone podzolization, are leached white, and are easily mapped. Near the present coast a peat/lignite bed, at a depth of 80 m, represents the IIIb highstand surface (oxygen isotope stage 3), indicating that 40 m of subsidence has occurred in the last 40 ka. In the alluvial valley, gravel dominates the base of an incised-valley fill 10 km wide and 45 m thick and is overlain by a fining-upward succession, the upper part of which is tidally influenced. Eroded Pleistocene terraces, mantled with thick peat, flank the Recent incised-valley fill. Within interfluve areas in the landward one-half of the northeast delta plain and adjacent coastal plain, thick Recent peat deposits (> 10 m) rest directly on or within a few meters of the buried remains of Pleistocene sediments. These peat deposits began accumulating between 7.3 and 5.8 ka as the rate of sea-level rise slowed. Recent siliciclastic sediments laterally adjacent to these peat deposits are composed of tidally influenced sands, silts, and clays. The seaward one-half of the northeast delta plain, delta front, and prodelta are composed of a seaward-thickening wedge of siliciclastic sediment up to 40 m thick that has accumulated in the last 5 ka. The base of the wedge is marked by a gravel lag that immediately overlies a rooted, yellow-brown alluvial soil. Siliciclastic sediments in this wedge consist of delta-front and prodelta clays and silts, delta-front distributary-mouth sands, and shoreline sands. Young (< 5 ka), reduced-thickness (< 10 m) peat deposits lie conformably on top of this wedge in this part of the delta plain. In contrast, the southwest part of the delta plain is not underlain by shallow-depth Pleistocene sediments, and it started to prograde into the South China Sea prior to 7 ka; its surface is dominated by beach ridges and gley soils mantled by mangrove– Nipa vegetation.

Abstract The Quaternary Baram Delta is more than 1 km thick on the outer continental shelf of Brunei, and includes mud-prone highstand delta lobes, sand-prone lowstand shelf-edge deltas, incised-valley fills, and transgressive sheet-like deposits lying on wave-cut ravinement surfaces. The shelf break is defined by a prominent fault scarp ∼ 130 m below sea level. Beyond, the seabed descends at an average of 2– 3° to the Borneo Trough at depths exceeding 2750 m. Uppermost Quaternary units thicken by a factor of 2–5 across en echelon shelf-edge growth faults, then progressively thin seaward as an extensive prodelta wedge that drapes irregular slope morphology. The slope owes its rugged relief to gravity-driven growth faulting, mud diapirism, cutting of submarine canyons, sediment sliding, and construction of levees alongside sporadically active turbidity-current channels that head in the region of shelf-edge deltas. During highstands, muddy lobes of the Baram Delta repeatedly extended to the upper slope as a consequence of high sediment yields and narrow shelf width. During the last lowstand, the Baram River cut an incised valley across the narrow shelf so that its muddy sediment load largely bypassed the continental margin through a major canyon. Delta deposits in the canyon head collapsed to generate a debris-flow complex that buries much of the canyon floor and that extends over large areas of the lower slope and Borneo Trough. This paper demonstrates that thick, extensive prodelta muds and associated turbidites can blanket basin slopes even during highstands in areas with high sediment supply and narrow shelves. Conversely, during lowstands, the suspended load of deltas can largely bypass the continental margin through incised valleys and canyons, resulting in reduced sedimentation on basin slopes.

Abstract The modern deltas of NW Borneo have long been regarded as wave-dominated, as typified by the Baram River Delta. However, the sedimentary facies associated with several modern deltas within Brunei Bay are strongly tide-dominated. A notable example is the Trusan River Delta, which occurs within a subsiding sub-basin and has an intertidal morphology and facies distribution that indicates tidal dominance, although the shoreline geometry suggests significant fluvial influence. The succession becomes sandier upward from embayment muds through mixed sand and mud flats to tidal-channel and bar sands near mean sea level, and then fines upward to tidal sand flats and finally mud flats near the high-tide shoreline. Tide-dominated sediments also are common in the outcropping strata of the mid-Miocene and younger Belait Formation. The Seven-Up Beach succession is interpreted as the distal ends of progradational lobes of a tide-dominated delta that coarsen upward from brackish-water mudstone through muddy and sandy tide-dominated mouth-bar deposits to tidal-channel and bar sandstones. The tidal-flat and tidal-channel and bar sandstones and interbedded tidal-flat mudstones of the Jalan Sungai Akar succession are more proximal deposits of a similar delta. Examples of distributary channels that eroded into lobe deposits during a relative sea-level fall and were backfilled during the subsequent relative sea-level rise are exposed in the Tanjong Batu succession. All three outcropping successions are strongly aggradational. A model for tide-dominated deltas in NW Borneo derived from the modern facies distribution and outcrop stratigraphy indicates that stratigraphic architecture is controlled primarily by the interaction of tectonic and compaction-driven subsidence, a relatively high rate of sediment supply, and basin hydrodynamics. The result is a largely aggradational, sandy intertidal succession on the delta plain and muddy delta-front deposits, with aggradational stacking of individual delta lobes.

Abstract Late Quaternary stratigraphy of the shelf over which the modern tropical Mahakam River delta is prograding indicates an alternating dominance of shelf carbonate sediments and siliciclastic deltaic deposits. Control of these major facies shifts is in response to high-frequency cyclic relative sea-level changes. Seismic facies and stratigraphy of the two latest Pleistocene depositional episodes reflect interplay of deltaic and shelf processes as well as accommodation during the last two glacio-eustatic cycles. Over 3000 line km of high-resolution seismic and side-scan sonar data calibrated to 380 bottom samples and 97 vibracores amd piston cores help characterize seismic facies of the Holocene and latest Pleistocene shelf deposits, which have been influenced by a strong north-to-south flowing current from the Makassar Strait. Holocene prodelta sediments are confined to the inner shelf of the northern and central sectors of the delta front, but they are skewed to the south, creating a broad facies tract in the southern sector. The middle and outer parts of the shelf are dominated by mounded topography characterized by both individual and aggregate bioherms. Mound relief varies from a few meters to over 30 m, but the average is about 20 m. This topography is locally accentuated by intervening erosional lows. Cores through the upper 6 m of these features indicate that the mounds are constructed of flakes of the calcareous green alga Halimeda in a matrix of foraminifera-rich terrigenous mud. The Halimeda bioherms are established on a ravinement surface formed during the Pleistocene to Holocene transgression. Inner-shelf bioherms are now being slowly buried by the Holocene advance of the Mahakam delta. Below the Pleistocene to Holocene ravinement surface are the deposits associated with the preceding relative sea-level fall. Deposits defining this falling-stage systems tract and associated lowstand systems tract consist of entrenched fluvial networks, incised-alluvial-valley fill, aggraded delta-plain and platform deposits, and prograded delta lobes. Pleistocene deltaic deposits have prograded to the shelf edge along the entire shelf–slope break. Over 50% of the stratigraphy in the lowstand systems tract is related to aggradation during the lowstand turnaround. The Pleistocene deltaic deposits bury massive carbonate bioherms and aggregates of bioherms that mimic their Holocene counterparts in morphology and scale. A prominent ravinement surface underlies the bioherm facies, and a maximum flooding surface runs through these features.

Abstract The delta of the Fly River is tide dominated, both morphologically and sedimentologically: the spring tidal range reaches 5 m, with near-bed tidal currents that commonly exceed 1 m/s, whereas wave influence is minimal, except during the monsoon period, when winds blow onshore. The delta plain displays a classic funnel-shaped geometry, with three main distributaries that flare seaward from a common bifurcation point 110 km inland. Of these, the southernmost is the main route for sediment export; the northernmost channel is effectively abandoned at present and is experiencing widespread erosion by tidal currents. Despite the net export of mud and sand, mutually evasive tidal-current patterns have created a series of elongate tidal bars within both active and abandoned distributary channels and in the distributary-mouth-bar area. The later is a zone of bedload convergence, with net seaward transport on its landward side and net landward transport on its seaward flank. This limits the offshore movement of sand. The deposits of the Fly delta are dominated by mud, because the river flows along the low-gradient axis of the foreland basin before reaching the sea, causing deposition of most sediment coarser than fine to very fine sand farther inland. Distributary-channel bases are floored by a thin unit of cross-bedded and rippled sand and mud-pebble conglomerate. These are typically overlain abruptly by mud deposits formed by dense fluid-mud bodies that form in the channel bottoms during spring tides. The mud layers in this facies are anomalously thick (commonly > 1 cm), and channel-floor deposits are characterized by interbedding of the coarsest and finest sediments. Above these muds, the sediments are pervasively heterolithic and show a net upward coarsening to about the mid-depth level on the tidal bars because of the thinning of the mud layers. These bars may contain 50% (or more) mud and display lateral-accretion bedding. Bioturbation is scarce to absent. The sediments then fine upward into the intertidal zone. Clear indications of a tidal origin for the heterolithic stratification are relatively uncommon, although tidal rhythmites are present locally, including within active channels. The mouth-bar deposits are predominantly sand and also contain lateral-accretion bedding. The delta-front facies are heterolithic, with both millimeter- and decimeter-thick sand/mud alternations, all with a limited degree of bioturbation. The prodelta consists of biologically homogenized mud. The stratigraphic organization of the deposits reflects the fact that the delta plain aggraded vertically during the last part of the postglacial sea-level rise, producing a complex stack of channel deposits, followed by progradation of the mouth bars by as much as 40–50 km.

Abstract The Holocene Mekong Delta is a good example of a delta that has evolved from a tide-dominated to a tide–wave mixed-type delta in its shape, sediment facies, and progradation rate during the regression after the maximum flooding during the Holocene. This evolution indicates that tide-dominated deltas developed well in the early phase of the highstand systems tract and that they changed into more wave-dominated deltas in response to subsequent regression. The Mekong Delta, which is located at the southeast tip of the Indochina Peninsula, is one of the largest deltas in Southeast Asia, with a large delta plain ranked third largest in the world. The present Mekong Delta is classified as a tide-dominated delta, but close to a wave-dominated delta, in the triangular deltaic depositional classification system. Two rotary-drilled cores, VL1 and TV1, were obtained from the flood plain and strand plain, respectively, of the delta plain. Detailed core descriptions, high-resolution 14 C dating by accelerator mass spectrometry (AMS), and topographical analysis of the delta plain indicate that the Mekong Delta evolved from a tide-dominated delta from 6.5 to 2.5 ka to a tide–wave mixed delta from 2.5 ka to the present. The present delta is characterized by beach ridges on the strand plain, dominantly wave-influenced sediments, and a low progradation rate due to longshore sediment dispersal by waves. As the sea level rises, marine flooding causes coastal inundation and the formation of embayments, which amplifies tides and shelters deltas from waves, so that they are more tide dominated. However, as a delta progrades and its shoreline migrates seaward, the resultant smoothed coastline, narrow shelf, and increased wave energy create a more wave-dominated coastal setting. This change will become more pronounced as the delta continues to prograde towards shelf margins.

Abstract The Sunda Shelf was extensively exposed during the last glaciation. As a result of the very low shelf gradient, this area is ideal for detailed studies of the sedimentary system and variations of coastal environments in response to sea-level changes. Thirty-six sediment cores from a transect inside the main paleo-valley of the North Sunda River on the middle Sunda Shelf were analyzed by sedimentological and geochemical methods to determine the facies associations. AMS 14 C dating of plant material and benthic foraminifera allowed detailed stratigraphic correlation offshore Malaysia for the first time. The drastic environmental transformations from terrestrial to marine conditions and the fate of the paleo–river system on the shelf are also reflected in deep-marine sedimentary records of the southern South China Sea. Large foresets of a delta system extended basinward following sea-level lowering before the Last Glacial Maximum. A widespread marshy soil formed immediately after exposure, and sediment bypassing dominated during the lowstand. The subsequent sea-level rise caused a stepwise submergence, controlled predominantly by the local morphological conditions. Drowning was restricted mainly to the North Sunda River valley up to 13.5 thousand calibrated years (ka). After 13.5 ka, flooding of the extended Sunda plain (≤ -70 m below modern sea surface) caused a loss of the upper course of the river system combined with a sudden interruption of the terrigenous supply. The transgressive deposits, which were formed during the sea-level rise, were closely related to the rapidly migrating paleo-shoreline. A fully marine regime of condensed muds was established in Holocene times, reflecting the completely new hydrographical conditions due to the final dissection of the remaining fluvial structures, and the opening of the Karimata Strait in the South. Three stratigraphic units, which are reliably dated offshore for the first time, are connected to regressive (-lowstand), transgressive, and stable phases. They are separated by stratigraphically important discontinuities. These units are correlated to the established Malaysian stratigraphy onshore referring to the transitional unit , the alluvial complex , and to the younger sedimentary cover .

Abstract The seven seismic units recognized in the Molengraaff paleo-delta on the Sunda Shelf underlying the postglacial unit can be interpreted as prograding shelf-margin lowstand wedges deposited during forced regressions. Speculative age estimates of these units were made by correlating the interpreted lowstand wedges with the SPECMAP δ 18 Ocurve. This correlation suggests that the oldest unit reaches back to 570 ka, that the mean sedimentation rates during glacial times are comparable to modern prodelta rates, and that the tectonic subsidence rate of the outermost shelf depressions during the latest Pleistocene was of the order of 27 cm/kyr. Subsidence (which was differential) also occurred in postglacial times. The rate at the shelf edge is of the order of 2.5 m/kyr, and the differential displacements are taken up by growth-fault activity. The outer Sunda Shelf was the delta plain of the Molengraaff River system during the last glacial, during which major fluvial entrenchment did not take place because of the low gradient. Sand ridges formed by transgressive reworking of Late Pleistocene material mark the modern water-depth range of 135–145 m. Mass wasting and fan deposition are important processes on the continental slope during regressions and lowstands.

Abstract Two main deltaic systems, Champion and Baram, filled in much of the Brunei shelf during the Late Miocene, Pliocene, and Pleistocene. “Fourth-order” deltaic sequences, commonly 50–100 m thick on the outer shelf, have been identified in seismic data, wireline well logs, and biostratigraphic data. Average duration of each Pliocene sequence is between 100,000 and 200,000 years. Tide- and river-dominated deltas were common in embayments formed during late stages of transgression and early highstand, but wave-dominated deltas were prevalent in middle to late highstand parts of sequences. During highstands of sea level, shales and siltstones dominated at shelf margins, whereas reservoir-quality sandstones occur 1–3 km or more landward of the contemporaneous shelf margin. From the Late Miocene through the Pleistocene, the Champion and Baram deltas prograded over thick mobile shales. The Champion delta (to the northeast) prograded approximately 40 km during the Late Miocene and 12 km in the Pliocene. The Baram delta (to the southwest) prograded approximately 40 km during the Pliocene. Delta progradation initiated movement on counter-regional growth faults on the upper slope that continued after the shelf margin prograded across those faults. The Frigate counter-regional fault system had 1–2 km of throw during the Pliocene and Pleistocene along the eastern part of the outer shelf. At the same time, much slower subsidence and even uplift occurred around diapiric shale ridges on inner shelf. Down-to-the-basin normal faults with 1–2 km of throw formed on the western part of the outer shelf (Baram delta) during the Pleistocene. Anticlinal folding and toe thrusts occurred on the middle and lower slope during times of major growth faulting (down-to-the-basin and counter-regional) on the outer shelf.

Abstract Heterogeneous sandstones in upper to middle Miocene lobes of the Mahakam Delta are prolific hydrocarbon reservoirs in the lower Kutei Basin, offshore East Kalimantan, Indonesia. At Attaka and Serang fields, more than 99 million cubic meters (627 million barrels) of oil and 37 billion cubic meters (1.3 trillion cubic ft) of gas have been produced from sandstones that we interpret as delta-front bars and tidal/fluvial distributary channels. Sand bodies of the modern Mahakam delta are analogs for many of these reservoirs. Delta-front bars are burrowed to laminated, fine-grained sandstones that form equant to somewhat dip-elongate bodies that range in thickness from < 1 m to 5 m and may exceed several kilometers in width. Cross-stratified, coarse- to fine-grained tidal/fluvial distributary channel sandstones are 3 to 17 m thick and narrow (< 1.5 km wide). Distributary-channel sandstones are typically highly porous (20–35%) and permeable (100–10,000 md), although tidal distributaries exhibit permeability heterogeneity, due to mud drapes and local burrows. Delta-front sandstones, although areally extensive, have generally poorer reservoir quality than the distributary channel sandstones ( k = < 0.1–1000 md; porosity = 10–25%). Also, the delta-front sands exhibit major centimeter- to decimeter-scale variations in permeability, which are related to variations in clay content and intensity of burrowing. Stacked, distributary-channel reservoirs are especially well developed in the Upper Miocene “Main Deltaics” interval, which we interpret as a succession of lowstand deltaic lobes. The coarsest-grained and thickest deltaic sandstones typically accumulate during relative lowstands, times when deltas have prograded long distances across marine shelves. Such lowstand sandstones also form the most porous and permeable hydrocarbon reservoirs. Thin-bedded sandstones and burrowed sandstones are common in distal deltaic deposits, which were probably deposited during times of somewhat higher relative sea level. Such sandstones may form low-resistivity reservoirs, which can, if recognized, contribute substantially to hydrocarbon production.

Abstract Mutiara field produces hydrocarbons from middle Miocene fluvio-deltaic successions within the Kutei Basin, East Kalimantan, Indonesia. In strata exposed over the doubly plunging Sanga-Sanga and Samboja anticlines, a large number of cores and downhole logs provide an excellent opportunity to integrate surface and subsurface data to improve reservoir characterization for exploration and development. Sedimentary facies and paleocurrent analysis were used to gain insight into the distribution of the dominant channel deposits within the succession. Channelized sandstone bodies identified in outcrops and cores constitute the main hydrocarbon reservoirs. They commonly comprise single-story distributary-channel sandstones and occasionally multistory alluvial-channel sandstones. Paleocurrent analysis revealed the distributary channels flowing in an overall southward direction, roughly parallel to the strike of the anticlines, while two of three multistory channels trend northeast. The third multistory channel has a strong westward flow direction, which might indicate a valley incision of a previously more sinuous channel. The orientation of the single-story distributary channels can be explained as a result of active tectonism during the middle Miocene. Incision of the Mahakam River into the uplifting hinterland means that a point source of sediment supply has existed to the north of Mutiara Field since middle Miocene times. Growth of anticlines through regional inversion of older, extensional basement faults has restricted the eastward progradation of the paleo–Mahakam Delta. As a result, the delta distributary channels and delta progradation was merely towards the south and north, parallel to depositional strike, and not perpendicular, as commonly thought. In addition, low directional variance among the single-story and multistory channels suggests that the paleo–Mahakam Delta comprised low-sinuosity channels, which has strong implications for the exploration of stratigraphic traps. Periods of dominant alluvial sedimentation produced roughly west–east striking multistory channels, which can sometimes be linked to incised-valley fills. These sandbodies might have had sediment sources to the southwest of the Mutiara field, and thus belong to a totally different fluvial system.