Bioturbated Sandstone Research Articles

Facies analysis and sequence stratigraphy of shallow marine, coarse‐grained siliciclastic deposits in the southern Utsira High: The Late Jurassic intra‐Draupne Formation sandstones in the Johan Sverdrup Field (Norwegian North Sea)

AbstractThin, condensed coarse‐grained shallow marine successions can be difficult to describe and interpret, especially in the subsurface since the recognition of finer‐grained intervals, typically associated with sequence stratigraphic surfaces, is challenging. This lack of mudstones and siltstones means that they also typically make excellent reservoir intervals. The Oxfordian to Volgian intra‐Draupne Formation sandstones in the Johan Sverdrup Field, southern Utsira High, represent such a system. This study presents a new sequence stratigraphic model for the Johan Sverdrup Field that unravels the detailed depositional history of the succession and places its formation within a regional Late Jurassic tectonostratigraphic framework. The intra‐Draupne Formation sandstones comprise four parasequences deposited following a regional Kimmeridgian marine flooding event. Sediments were mainly supplied through West‐derived fan deltas from the Haugaland High and NW‐SE‐directed tidal currents reworking the Augvald Graben and the Avaldsnes High at the East. The oldest parasequence shows a distinctive suite of facies consisting of fine‐grained and mud‐rich bioturbated sandstones deposited in a semi‐restricted lagoon. Subsequent parasequences lack fine‐grained sediments and are dominated by bidirectional cross‐stratified, very coarse‐to coarse‐grained sandstones and gravels deposited in a tidal strait. A progressive reduction of fault‐related subsidence in the Middle Volgian along with Late Volgian‐Ryazanian sea‐level rise and inversion of pre‐existing structures promoted backstepping of the feeder systems, sediment starvation and the progressive deposition of the black and green‐red shales of the Draupne and Asgard formations. The results of this study account for features previously unidentified in the Johan Sverdrup Field and which have implications for understanding the deposition of coarse‐grained shallow marine successions around the Utsira High and other transgressed basement highs.

Lithology and Lithofacies Characteristics of Deeply Buried Clastic Reservoirs in Alternating Wet-arid Climate and its Hydrocarbon Significance

There have been series of HPHT (high temperature and high pressure) reservoirs with good quality and high productivity discovered in the Triassic Skagerrak Formation in the Central Graben of the UK North Sea. The bottlenecks for further E&P activity are the complicated depositional facies and high reservoir heterogeneity. The main reservoir space is a mixed porosity network composed of primary pores, micro-pores, secondary pores, secondary micro-pores and micro-fractures. Reservoirs of the Skagerrak Formation have greatly varied lateral connectivity as well as poor vertical connectivity. In the case of sufficiently high pressure differentials, hydrocarbon can be fed from high-potential area to low-potential area. There is no simple linear relationship between depth and reservoir quality. The primary depositional texture (grain size/sorting/clay content, etc.) linked to facies has a fundamental control on reservoir quality. Clean and well sorted cross laminated sandstone has the best reservoir quality, while the reservoir quality of planar laminated sandstone decreased due to increased degree of compaction caused by mica drapes. The reservoir quality of the fine-grained ripple cross laminated sandstone is even poorer than that of the planar laminated sandstone, due to abundant mica and clay drapes. Mottled bioturbated sandstone is fine grained with high clay content, which is more muddy because of containing burrows and insect larva. Pedoturbated sandstone has the poorest reservoir quality due to severe dolomite cementation. The dominant detrital material of the Skagerrak Formation is feldspar that provides abundant secondary porosity by alteration and dissolution. Chlorite coats are commonly developed and stained with residual oil or bitumen, which clearly inhibits quartz cementation and feldspar overgrowth. The presence of these clay coats contributes to the preservation of anomalously high porosity at depth in the cleaner and coarser grained fluvial channel facies. The Skagerrak formation facies is a dryland fluvial-lacustrine depositional system which is controlled by fluctuations of depositional base level due to salt mobilisation. The Skagerrak formation consists of several cycles that are vertically separated by lacustrine mudstone. The reservoir permeability system is composed of active fluvial channel sandstones, terminal splay sandstones, abandoned fluvial channel sandstones, minor channel sandstones connecting fluvial channel and terminal splay, as well as low-permeability faults, while the active fluvial channel sandstone is the “sweet spot”. The analysis of controls on deeply buried reservoir quality is significant to precisely predict “sweet spot” in the Skagerrak Formation.

An integrated workflow for seismic interpretation, petrophysical and petrographical characterization for the clastic Mangahewa reservoir in Pohokura gas field, Taranaki Basin, New Zealand

This research aims at delineating the main structural elements controlling the geologic setting of Taranaki Basin in New Zealand which in turn control the hydrocarbon potential of this basin and in particular of Pohokura Field which is considered one of the main gas-producing fields in the basin. This can be achieved utilizing the seismic data including three different marine seismic data sets covering 514.6 km2 with 12.5 m horizontal resolution. A synthetic seismogram and a set of seismic attributes were created to achieve this target. In addition, an integrated reservoir characterization study is applied to the Mangahewa reservoir in the Pohokura Field. This integrated characterization includes a detailed petrographical lithofacies analysis, well logging, and core analysis studies. Based on the petrographical study, eight lithofacies are described in Pohokura Field: 1) Laminated sandstone; 2) Massive sandstones; 3) Granular laminated sandstones; 4) Bioturbated sandstones; 5) Sand dominated and mud rich bioturbated sandstones; 6) Muddy bioturbated sandstone; 7) Laminated and rippled laminated mud rocks, and 8) Coal lithofacies which have been deposited in high energy shoreface setting. On the other side, the available well logging data includes caliper, GR (gamma-ray), sonic, neutron, density, and resistivity (shallow and deep resistivities). Logging interpretation enabled the estimation of the effective porosity (av. ∅ = 11.4%), hydrocarbon saturation (av. Shc = 62.87%), and the shale volume (av. VSh = 18.02%) with a total net pay thickness of all wells = 196 m. The available core data includes porosity, density, and permeability which are used to estimate the reservoir quality ranks; the effective pore radius (R35); the flow zone indicator, the discrete rock type (DRT), and the reservoir quality index (RQI). Considering the porosity, permeability, and the DRT values, the available core data are grouped into five reservoir rock types (RRTs) with the best reservoir quality assigned for the RRT1 (bioturbated, laminated and massive sandstones), while RRT5 has the lowest reservoir quality (muddy bioturbated sandstone, Granulated laminated sandstone, and laminated and rippled mudstone). This integrated study is applicable and extendable to the other basins in New Zealand and the southwestern Pacific. The proposed workflow is also applicable to the other analogs in the various basins elsewhere.

Assessing the impact of bioturbation on reservoir quality: Integrated spontaneous imbibition, pressure decay profile permeability and pore-throat characterization study of the Cretaceous Bhuj formation, Kachchh basin, India

The Lower Cretaceous Bhuj Formation in Kachchh basin is well known for its hydrocarbon potentials including proven gas reserves in Kachchh offshore. In the onshore these reservoir equivalent rocks outcrops and includes sandstones (quartz arenites), with higher degree of bioturbations. The present work, deals with determining of impact of variable bioturbation degree sandstones reservoir quality. The petrographic properties were integrated with spot permeability and spontaneous imbibition study to understand petrophysical contrast between burrow and non-burrowed part.In moderately bioturbated samples (BI-3), spot permeability for burrows was 18 times higher than non-burrowed matrix. While in higher bioturbated, samples (BI-4) the permeability contrast between burrows and non-burrowed area was moderately lower (10.26 times). Similarly in highly bioturbated sandstones of BI-5 the permeability contrast between burrow and non-burrow was less up to 5.45 times. Thus, highly bioturbated sandstones appears to be less heterogeneous as compared with moderately bioturbation sandstones. To support this study, qualitative and quantitative investigation of the pore network, pore structure and pore connectivity through Scanning Electron Microcopy (SEM) indicated a larger proportion of intergranular pores in burrow fill than in the surrounding matrix for all types of bioturbation classes. High pressure Mercury Injection (HPMI) data for quantitative validation of these inferences also estimated a higher presence of connected macropore volume in burrowed regions of Bioturbation Index 4 and 5. In contrast, the Bioturbation Index-3 indicated highest connected mesopore volume for the non-burrowed area. The imbibition characteristic corroborated with the better pore connectivity and larger pore throat size in the burrow volume than the non-burrowed matrix estimated from other studies. The Bioturbation Index 4 and 5 had higher initial imbibition rates and imbibition capacities than Bioturbation Index-3. The well-developed and connected pores in the mesopore region tend to form a large transition zone, while a higher proportion of connected pore volume in Bioturbation Index 5 is exhibited in a higher late imbibition rate. In essence, a conceptual model is presented to describe spontaneous imbibition in bioturbated media, in order to understand the enhanced petrophysical quality in bioturbated sandstones.

Biogenically modified reservoir rock quality: A case from the lowermost member Paleocene Funing Formation, Gaoyou Depression, Subei Basin, China

Bioturbation can influence reservoir quality and is consequential to the producibility of a reservoir. The study of samples from the lowermost member of the Paleocene Funing Formation (E1f1), Gaoyou Depression, Subei Basin, shows how bioturbation affects reservoir quality. Techniques used to study the samples include petrography, pressure decay porosimetry, pulse decay permeametry, Field Emission Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy. Sample A is intensely burrowed by Taenidium, Scoyenia, Skolithos, Palaeophycus, and other trace fossils. Increased isotropy in sample A contributes to cleaner well-sorted burrows, relative to its surrounding matrix, and 67.18% augmented burrow porosity. Taenidium and Palaeophycus in sample B indicate 20.23% improved burrows porosity. Plant debris and/or root traces in sample C have a 3.68% reduction in porosity. In samples A and B, the arithmetic mean of permeability describes all horizontal fluid flows within burrows. In sample C, the geometric mean of permeability describes the fluid flow in all directions. Porosity is ≤ 11.2%, permeability ≤1 md in samples, and sample C log-derived porosity ≤0.33%. This study demonstrates that bioturbation together with depositional factors (sorting, grain size distribution, and mud-matrix/burrow content) and diagenetic modifications (albitization, compaction, dissolution, kaolinization, and precipitation) control the quality of the high to intensely bioturbated (Bioturbation Index 4 to 5; 61–99 vol %) sandstone and siltstone reservoir facies of the E1f1.

Evaluating cementation factor for bioturbated sandstones, Bhuj Formation, Kachchh Basin, India: a novel approach using triple-porosity model

Cementation factor has a crucial role in evaluating volumetric reserves for heterogeneous clastic reservoir. Bioturbation is a prominent source of heterogeneity existing in hydrocarbon-bearing formation across the world. The effect of bioturbation on the petrophysical properties of the clastic has not been comprehensively understood yet, especially its effect on cementation factor estimation. The precise description of such properties is beneficial in developing better volumetric estimates and optimizing field development planning. The present work discusses an approach to characterize cementation factor in bioturbated clastic Bhuj Formation, Kachchh Basin, India. In order to ascertain the cementation exponent for bioturbated media, an improved triple-porosity model is proposed by quantifying the total porous volume into three components, namely matrix 1, matrix 2 and bioturbated region. An equivalent resistance model is then developed by identifying the series and parallel relation between the two matrix components and the burrow. The experimental observations of electrical response indicated large variation in resistivity in vertical cores compared to horizontal cores. The decreased resistivity is due to the vertically aligned burrow which due to their enhanced porosity provides less tortuous path for current to flow. The triple-porosity model is validated with different porosity value of matrix 1, matrix 2 and burrow, and a cementation factor of 2.8 for higher bioturbated samples and 3 for lower bioturbated samples is obtained. This validates the previous understanding of enhanced flow and storage property and thereby produces a consistent way to evaluate total cementation factor. The correct estimation of cementation factor helps in precise evaluation of water saturation in bioturbated reservoirs.

3D geo-cellular static virtual outcrop model and its implications for reservoir petro-physical characteristics and heterogeneities

Geostatistical data plays a vibrant role for surface-based reservoir modeling through outcrop analogues, which is used to understand three-dimensional (3D) variability of petrophysical properties. The main purpose of this study is to improvise the surface-based 3D geo-modeling to demonstrate petrophysical characteristics and heterogeneities of Sandakan reservoirs, NW Borneo. We used point cloud data from Light Detection and Ranging (LiDAR) to build high-resolution virtual outcrop modeling (VOM) onto which we mapped 6 different lithofacies. Porosity and permeability of core plugs were measured to determine the average variance of petrophysical properties for each lithofacies. By utilizing the integration of VOMs analogues and petrophysical properties in Petrel™, we demonstrated the distribution and associations of all lithofacies in pseudo wells that have inherent thin beds heterogeneities in 3D geo-cellular model. The results concluded that the heterogeneity of thin beds in lithofacies is dependent on porosity and permeability with input dataset. According to the final model, cross-bedding sandstone (CBS), hummocky cross-stratified sandstone (HCSS) and trough cross-bedding sandstone (TCBS) show good reservoir quality due to high porosity ranging from: 25.6% to 20.4% and, 19.3%–14.5%, and permeability ranging from: 74.03 mD to 66.84 mD and, 64.86 mD to 21.01 mD. In contrast, massive to weak laminae sandstone (MWLS) and bioturbated sandstone (BS) show fair to poor reservoir quality, caused baffling of surrounding mud sediments in the reservoir lithofacies. Results also revealed that LiDAR based VOM with petrophysical properties can significantly reduce the risk and minimize the cost of reservoir modeling in petroleum industry.

Regional stratigraphy and proximal-to-distal variation of lithology and porosity within a mixed carbonate-siliciclastic system, Mississippian strata of northern and central Oklahoma

Mississippian carbonate and silica-rich reservoirs of northern and central Oklahoma formed along a regionally extensive carbonate ramp to basin transect. The stratigraphy, lithology, and porosity characteristics of the Mississippian Meramec and Osage series significantly vary as older ramp carbonates prograde southward and transition into younger calcareous and quartz-rich silt deposits of the Anadarko Basin. Lithofacies identified within the northern carbonate-dominated portion of the system commonly include altered chert, skeletal grainstones, peloidal packstones-grainstones, bioturbated wackestones-packstones, bioturbated mudstones-wackestones, glauconitic sandstones, and siliceous shale. Lithofacies within the southern siliciclastic-dominated portion of the system include structureless to bioturbated sandstones, siltstones, and laminated mudstones, each with varying degrees of carbonate content. We have grouped these core-based lithofacies into dominant lithologies/rock types, which tie to well-log properties. Electrofacies classification methods including the artificial neural network (ANN) and k-means clustering predict lithologies in noncored wells. ANN yielded the highest overall prediction accuracy of 85% for lithologies. Core, well log, and lithology log data establish the regional stratigraphic framework. The Mississippian interval of interest subdivides into 16 stratigraphic zones. A depositional-dip-oriented cross section and the associated reservoir models illustrate the proximal to distal and stratigraphic variability of the lithology and porosity. Lithology trends moving from north to south, from older to younger strata, reveal a carbonate-dominated succession capped by diagenetically altered chert northward shifting into a siliciclastic-dominated interval, which increases in clay content southward. Northward, prospective conventional reservoirs developed near cycle tops within diagenetically replaced cherts and cherty limestones associated with subaerial exposure and sea-level fluctuations. Southward, higher total porosity associates with increased clay content linked to the suppression of calcite cement, forming prospective unconventional targets near the bases of depositional cycles.

Sedimentary facies and environments of the sedimentary fill of Southern Bida Basin, Nigeria

Six lithofacies were identified in the Lokoja Formation, Southern Bida Basin: fanglomerate/ conglomerate lithofacies (Gmc), fine to coarse-grained ferruginized weakly cross-bedded, pebbly sandstone lithofacies (Scx), fine to coarsegrained sandstone lithofacies (Sfc), silty claystone lithofacies (Csm), siltstone lithofacies (Slt) and lateritic ironstone lithofacies (Ilt). These were grouped into three lithofacies associations viz: alluvial fan, braided river channel, floodplain lithofacies association. Nine lithofacies were identified in the Ahoko Formation. These are: black-dark grey carbonaceous shale lithofacies (Shc), bioturbated ripple-laminated siltstone lithofacies (Sbr), poorly cross-laminated claystone lithofacies (Cxl), concretionary/nodular ironstone lithofacies (Icn), medium to coarse-grained sandstone lithofacies (Smc) fine grained, well-sorted, friable bioturbated herringbone cross-bedded sandstone lithofacies (Sxf), massive brownish claystone lithofacies (Clm), massive claystone with lateritic ironstone lithofacies (Cli) and lateritic ironstones lithofacies (Ilt). These have been grouped into three lithofacies associations viz: shallow marine lithofaciesassociation, tidal-intertidal flat lithofacies association and floodplain lithofacies association. Similarly, three lithofacies were identified in the Agbaja Formation and have been grouped into two lithofacies association. These are: fine to medium-grained sandstone ironstone interbedded lithofacies (Sti), oolitic–pisolitic ironstone lithofacies (Iop) and concretionary ironstone lithofacies (Icr). The lithofacies associations are: tidal-intertidal flat lithofacies association and shallow marine lithofacies association. Result of lithofacies analysis helped in interpreting the depositional environments. The Lokoja Formation is a product of a fluvial dominated alluvial system from debris/gravity flow in alluvial fan. This developed further into braided river channels and later meandering river during the closing stages. Sediments of the Ahoko Formation were deposited in tidal/intertidal flats and shallow marine environments while sediments of the Agbaja Formation were produced by a shallow marine system with a high tidal influence.

Middle Permian trace fossil assemblages from the Carnarvon Basin of Western Australia: Implications for the evolution of ichnofaunas in wave-dominated siliciclastic shoreface settings across the Permian-Triassic boundary

Guadalupian (middle Permian) ichnoassemblages are described from the Mungadan Sandstone Formation (Kennedy Group), Merlinleigh sub-basin, Carnarvon Basin of Western Australia. In this formation, we recognize five facies (pebbly sandstone, intensely bioturbated sandstone, medium- to coarse-grained cross-stratified sandstone, fine-grained sandstone, and planar laminated siltstone), assigned to two facies associations, interpreted as the deposits of wave-dominated upper shoreface and lower shoreface settings. Ichnoassemblages, which contain 23 ichnospecies in 18 ichnogenera, are assigned to the Skolithos and Cruziana ichnofacies, as well as mixed types, and include representatives of cubichnia, repichnia, pascichnia, fodinichnia, domichnia, and agrichnia, indicating diverse trace-making behaviours. High burrow-density with monospecific occurrences of Skolithos indicate R-selected life strategies and burrow production over a short time period. The anomalous occurrence of Paleodictyon implies that the trace-making organisms of graphoglyptids could inhabit resource-rich shoreface environments as well as resource-limited deep sea niches. To better understand ichnological evolution across the Permian-Triassic (P-Tr) boundary, a database of Permian to Middle Triassic ichnoassemblages was also compiled for wave-dominated shoreface settings (WDSS) globally, and three ichno-ecological measures (ichnodiversity, ethological category, and ichnodisparity) were evaluated. Ichnodiversity shows a stepwise decline in ichnotaxa through the Permian, a pronounced increase in the Early Triassic, and a second phase of decline towards the Middle Triassic. Ethological data exhibit a different trend, with mean and median values dramatically dropping to a late Permian nadir, followed by a significant increase across the P-Tr boundary, culminating in an Early Triassic acme. The globally depauperate nature of late Permian ichnoassemblages may reflect sampling bias or decreased preservation potential resulting from abundant benthos and well-developed mixed layer in the seafloor. The presence of anomalously diverse Early Triassic ichnoassemblages may be due to the WDSS habitat providing a ‘habitable zone’ for trace-making organisms to survive when most marine environments were stressful in the aftermath of the P-Tr mass extinction. We emphasise that caution is needed in interpreting ichnodiversity variations across the P-Tr mass extinction event, as it may reflect shifting of trace-making behaviours of the same animals, or replacement of different animals with the same behaviours. • A new Guadalupian ichnofauna comprising 18 ichnogenera is described; • The Carnarvon ichnofauna represents the Skolithos and Cruziana Ichnofacies; • Paleodictyon occurs in wave-dominated siliciclastic shoreline (WDSS) settings; • The depauperate of ichnotaxa in the late Permian reflect sampling bias or decreased preservation potential of ichnofossils. • The diverse ichnoassemblages in WDSS settings during the Early Triassic results from ‘habitable zone’.

Characteristics of a Campanian delta deposit controlled by alternating river floods and tides: the Loyd Sandstone, Rangely Anticline, Colorado, U.S.A.

ABSTRACTThe Campanian Loyd Sandstone Member of the Mancos Shale (Loyd) along the Rangely Anticline, Piceance Basin, Colorado, contains a series of multi-meter-thick, heterolithic, coarsening-upward successions (CUSs) with internal clinoforms comprising low-angle-dipping (< 5°), parallel-laminated sandstone beds interbedded with bioturbated flaser-wavy-lenticular bedded sandstone and siltstone. Loyd clinoforms are delta foresets composed of prodelta and delta-front deposits, including mouthbars. Many CUSs exhibit scours filled with bioturbated sandstone, or interbedded bioturbated sandstone and siltstone or mudstone representing aggradational fill of subaqueous terminal distributary channels or the migration of mouthbars into channel scours. Mud drapes on sedimentary structures and mud rip-up clasts are extremely common. A high-abundance, high-diversity, trace-fossil assemblage includes vertical, 1–4-m-deep Ophiomorpha that may penetrate multiple bedsets of parallel-laminated sandstones and highly bioturbated finer-grained interbeds. Bioturbation increases sandstone content in finer-grained interbeds, and provides sandy conduits that increase connectivity between beds.Although parallel-laminated sandstones volumetrically dominate Loyd delta clinoforms, they likely represent relatively short-term freshwater and sediment input during river flooding that produced delta-front turbidity currents. Interbedded finer-grained beds, mud-draped sedimentary structures, and the high-abundance, high-diversity trace-fossil assemblages record longer amounts of time during reduced fluvial discharge, tidal reworking of sediments, and intense bioturbation under marine salinities. Controls on the internal characteristics of the Loyd delta deposits include: 1) a high sediment influx into a relatively shallow marine basin protected from wave action during normal regression; 2) relatively short-duration, episodic freshwater and sediment discharge from distributaries that reduced salinities, deposited sand as turbidity currents, and promoted delta-front channelization; and 3) longer-duration periods of reduced discharge with deposition of finer-grained sediment, tidal reworking, and bioturbation of sediments under higher salinities. These controls combined to produce the heterolithic, highly bioturbated, river-flood delivered, and tidally modified clinoforms of the Loyd that differ from deposits typically considered to be classic examples of fluvial-flood-dominated or extensively tidally modified deltas.

Taphonomy of and new burrows from Oryctodromeus cubicularis, a burrowing neornithischian dinosaur, from the mid-Cretaceous (Albian-Cenomanian) of Idaho and Montana, U.S.A.

Vertebrate assemblages from the mid-Cretaceous (Albian-Cenomanian) Wayan Formation of Idaho and Vaughn Member of the Blackleaf Formation of Montana are dominated by the small burrowing dinosaur, Oryctodromeus cubicularis. Specimens can be assigned to three distinct taphofacies dependent on degrees of skeletal articulation and associated lithofacies. Taphofacies A comprises mostly to fully articulated specimens in bioturbated sandstones. Taphofacies B comprises partially articulated to associated collections of skeletal elements in pedogenic mudstones, siltstones, and fine-grained sandstones. Taphofacies C comprises isolated skeletal elements within the debris-flow deposit of the Robison Bonebed. Except for one associated nodosaur skeleton, Oryctodromeus is the only vertebrate represented by significant associations of skeletal elements or portions of articulated skeletal elements within the Wayan Formation/Vaughn Member units, and occurrences represent a paleocommunity dominated by a small cursorial and fossorial vertebrate. The co-occurrence of multiple individuals, representing presumed juvenile and adult growth stages, reinforce previous suggestions of parental care in this species. In addition, newly reported Oryctodromeus burrows compare in overall morphology and size with the burrow containing the holotype of Oryctodromeus. These data suggest the possibility that evidence for additional fossil taxa having had a communal and fossorial lifestyle may be discerned through further focused taphonomic investigations.

Shrimps and leaves: Phytodetrital pulses and bioturbation in deposits of a river-dominated delta (Middle Jurassic Lajas Formation, Neuquén Basin, Argentina)

The Middle Jurassic Lajas Formation of the Neuquén Basin (Argentina) records mainly deltaic sedimentation. However, the precise role of the depositional mechanisms involved is still controversial. Well core YPF Nq. Laj-8 records the overall vertical transition from delta-front mouth bars to delta plain. Discrete mouth-bar successions typically comprise a lower unbioturbated sandstone unit that passes upwards into a more intensely bioturbated sandstone unit. Phytodetrital material is abundant through the mouth-bar succession forming distinct layers in the lower units and becoming part of the lining of the burrow walls of Ophiomorpha irregulaire and Gyrolithes isp. in the upper units, representing two successive developmental stages of deltaic sandy mouth bars. Layers rich in phytodetritus exhibiting the same characteristics have been traditionally interpreted in outcrop as mudstone drapes generated by tidal action during slack-water periods. However, more recent research suggests that these layers can be best regarded as produced by river-fed flows, which is consistent with a river-dominated delta model.

Identification and characterization of low resistivity low contrast zones in a clastic outcrop from Sarawak, Malaysia

Zones with low resistivity low contrast (LRLC) are usually overlooked due to the absence of high-end well logging techniques in the older oil/gas fields. The Nyalau Formation outcrop (stratigraphically equivalent to offshore cycles I and II) from Sarawak, offers a good opportunity to identify and characterize LRLC zones through integrated approaches. Seven facies of the Nyalau Formation had been identified, which are: 1) low angle cross-bedded sandstone (CBSS), 2) laminated sandstone (LSS), 3) heterolithic sandstone (HSS), 4) bioturbated sandstone (BSS), 5) clay-rich sandstone (CSS), 6) mudstone (MS), as well as 7) trough CBSS (TCBSS). Apparent resistivity was determined for brine (60 kpm) saturated core plugs at a temperature of 75 °F by employing the sample core induced polarization technique. The measurement of resistivity was calibrated based on offshore salinity, as well as surface and subsurface temperature of the Sarawak. The geological and petrophysical characteristics were analyzed through field observations and lab analyses. The findings of the study revealed that the cutoff value of resistivity found at LRLC zones in Nyalau Formation was 12.6 Ω-m. Accordingly; LSS, BSS, and CSS facies appeared to derive from LRLC zones. The main causes that generate such LRLC zones are laminations, thin bedding, bioturbation, and clay minerals. The dominant effect portrayed by mixed layers of illite/smectite and smectite in LRLC zones could be attributed to their high cation exchange capacity (CEC). Two varying resistivity trends were discovered based on the variances detected in clay types for different facies. A negative correlation was found between resistivity and porosity within high resistivity zones, while a positive relationship within the LRLC zones. Disparity in the resistivity-porosity correlation is deduced to be associated with clay mineralogy. This study suggests that LRLC zones are expected to be present in shallow marine clastic reservoirs of offshore cycles (I and II), where the offshore reflects Nyalau Formation.

Reservoir heterogeneity of Agbada Formation sandstone from PZ-2 well and PZ-4 well, Niger Delta: implications for reservoir management

Sandstones of the Agbada Formation deposited in Eocene serves as the main hydrocarbon reservoirs of the Niger Delta Basin. PZ-2 well and PZ-4 well that penetrated sections of the formation were investigated for their reservoir heterogeneity, to understand the hydraulic flow units and better assessment of the reservoirs. Reservoir quality and pore throat sizes encountered in the sandstones vary from poor to excellent quality and macroscopic. Greater heterogeneities permeability and homogeneous porosity distribution occur in the bioturbated sandstone facies in the PZ-4 well and sandstone facies in the PZ-2 well. Four hydraulic flow units (FZ-1, FZ-2, FZ-3, FZ-4) and three hydraulic flow units (FZ-1, FZ-2, FZ-3) were recognized in the PZ-2 and PZ-4 wells respectively. These flow zones are characterised by good reservoir qualities and adjudged as a better reservoir candidate for probable prospects.

Bioturbation influence on reservoir rock quality: A case study of Well Bian-5 from the second member Paleocene Funing Formation in the Jinhu sag, Subei basin, China

The bioturbated horizons identified from Member Two of the Paleocene Funing Formation (E1f2) in the Jinhu sag, have not been previously assessed to ascertain their influence on reservoir quality. Owing to that, conventional thin section, pulse decay permeametry, pressure decay porosimetry, Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive X-ray Spectroscopy (EDS) methods were used to analyze the significance of bioturbation in controlling porosity and permeability on selected cored intervals from one well. Intensely bioturbated core slab samples A and B selected from Bian-5 Well are dominantly controlled by Ophiomorpha and Macaronichnus, and Macaronichnus, respectively. Results showed that Ophiomorpha and Macaronichnus in sample A exhibited sediment cleaning activity with better sorted grains in burrow fillings. Mud and clay were removed from the burrow filling and directed towards burrow lining and/or host sediment by burrowing organism resulting in well sorted burrow fill matter; the net effect, enhanced isotropy and increased porosity in burrows fills. Macaronichnus ichnofabrics in sample B also exhibited sediment cleaning activity causing reduction of local porosity in host sediment and an outcome of permeability improvement in burrows. Reservoir quality of bioturbated sandstone of Bian-5 well in the Jinhu sag is, therefore, dependent on activity of burrowing organism, burrow structure, bioturbation extent as well as the existence or nonexistence of burrow fill or lining.

High resolution facies architecture and digital outcrop modeling of the Sandakan formation sandstone reservoir, Borneo: Implications for reservoir characterization and flow simulation

Advances in photogrammetry have eased the acquisition of high-resolution digital information from outcrops, enabling faster, non-destructive data capturing and improved reservoir modeling. Geocellular models for flow dynamics with in the virtual outcrop in siliciclastic deposits at different sets of sandstone facies architecture remain, however, a challenge. Digital maps of bedding, lithological contrast, spatial-temporal variations of bedding and permeability characteristics make it more easy to understand flow tortuosity in a particular architecture. An ability to precisely model these properties can improve reservoir characterization and flow modeling at different scales. Here we demonstrate the construction of realistic 2D sandstone facies based models for a pragmatic simulation of flow dynamics using a combination of digital point clouds dataset acquired from LiDAR and field investigation of the Sandakan Formation, Sabah, Borneo. Additionally, we present methods for enhancing the accuracy of outcrop digital datasets for producing high resolution flow simulation. A well-exposed outcrop from the Sandakan Formation, Sabah, northwest Borneo having a lateral extent of 750 m was chosen in order to implement our research approach. Sandstone facies and its connectivity are well constrained by outcrop observations, data from air-permeability measurements, bilinear interpolation of permeability, grid construction and water vector analysis for flow dynamics. These proportions were then enumerated in terms of static digital outcrop model (DOM) and facies model based on sandstone facies bedding characteristics. Flow simulation of water vector analysis through each of the four sandstone facies types show persistent spatial correlation of permeability that align with either cross-bedded orientation or straight with more dispersion high quality sandstone (porosity 21.25%–41.2% and permeability 1265.20–5986.25 mD) and moderate quality sandstone (porosity 10.44%–28.75% and permeability 21.44–1023.33 mD). Whereas, in more heterolithic sandstone (wavy- to flaser-bedded and bioturbated sandstone), lateral variations in permeability show spatially non-correlated patterns over centimeters to tens of meters with mostly of low quality sandstone (porosity 3.4%–12.31% and permeability < 1 mD to 3.21 mD). These variations reflect the lateral juxtaposition in flow dynamics. It has also been resulted that the vertical connectivity and heterogeneities in terms of flow are mostly pragmatic due to the interconnected sandstone rather than the quality of sandstone.

Stratigraphy and age estimate of Neogene shallow marine fossiliferous deposits in Brunei Darussalam (Ambug Hill, Tutong district)

The Ambug Hill in Brunei is an exceptional geological site where a series of siliciclastic rocks crops out with some layers extremely rich in marine fossils. Such fossiliferous outcrops are extremely rare in the northern part of Borneo and their description is of primary importance as their fossil content can be used to correlate the regional depositional sequences with global biostratigraphic zonations. In this work we present for the first time a detailed sedimentary profile completed with Sr-isotopes and biostratigraphic dating. The succession is divided into four sedimentary units. The first unit comprises bioturbated sandstone followed by a second unit of clay-silt rich levels whose first 9.5 m contain rich marine fossil assemblages. Calcareous nannoplankton data indicate a Late Tortonian – Early Messinian (NN11) age, which is confirmed by Sr-isotope dating derived from bivalves giving a numerical age range from 8.3 to 6.2 My. After a major emersion surface, the third sedimentary unit of sand- and siltstone lacks suitable fossils for bio- and isotope stratigraphy. The age of emersion and the related sedimentary gap can be either correlated with the Me1 (7.25 My) or the Me2 (5.73 My) sequence boundary. A fourth, thin sedimentary unit is recognized on the top of the profile with silt- and claystone beds without age diagnostic remains and calcareous fossils.

An enhanced approach to predict permeability in reservoir sandstones using artificial neural networks (ANN)

A key challenge in the oil and gas industry is the ability to predict key petrophysical properties such as porosity and permeability. The predictability of such properties is often complicated by the complex nature of geologic materials. This study is aimed at developing models that can estimate permeability in different reservoir sandstone facies types. This has been achieved by integrating geological characterization, regression models and artificial neural network models with porosity as the input data and permeability as the output. The models have been developed, validated and tested using samples from three wells and their predictive accuracy tested by using them to predict the permeability in a fourth well which was excluded from the model development. The results indicate that developing the models on a facies basis provides a better predictive capability and simpler models compared to developing a single model for all the facies combined. The model for the combined facies predicted permeability with a correlation coefficient of 0.41 which is significantly lower than the correlation coefficient of 0.97, 0.93, 0.99, 0.96, 0.96 and 0.85 for the massive coarse-grained sandstones, massive fine-grained sandstones-moderately sorted, massive fine-grained sandstones-poorly sorted, massive very fine-grained sandstones, parallel-laminated sandstones and bioturbated sandstones, respectively. The models proposed in this paper can predict permeability at up to 99% accuracy. The lower correlation coefficient of the bioturbated sandstone facies compared to other facies is attributed to the complex and variable nature of bioturbation activities which controls the petrophysical properties of highly bioturbated rocks.

The stratigraphic architecture and depositional environments of non-marine carbonates from barremian-aptian pre-salt strata of the Brazilian continental margin

The stratigraphic architecture and depositional environments of non-marine carbonates from barremian-aptian pre-salt strata of the Brazilian continental margin