Fluvial Mudstone Research Articles

Quantifying a tide-dominated, wave-, and river-influenced delta in Miocene facies of the Niger Delta basin

ABSTRACT The modern Niger Delta has long been classified as a mixed tide-, wave-, and river-influenced delta. Still, no detailed studies have quantified the relative proportions of formative processes and facies. This work presents the first quantitative estimate of the relative influence of formative processes in Miocene deposits based on core data from the Greater Ughelli, Central Swamp, and Coastal Swamp depositional belts of the Niger Delta basin. Facies analysis of 288.2 m of core from four wells shows approximately 53% tidal facies, 33% fluvial facies, and 14% wave-formed facies, indicating deposition in a prograding tide-dominated, wave-, and river-influenced delta. Cores from the Greater Ughelli and Central Swamp depobelts exhibit coarsening-upwards, prograding deltaic facies successions overlain by fluvial mudstone. Tidal deposits in the Coastal Swamp depobelt show coarsening-upwards prograding deltaic facies successions with well-developed tidal bundles indicating seasonal deposition. The basal part of these facies successions also reveals repeated floods characterized by slump and load structures and dewatering features. Delta-plain and delta-front–prodeltaic facies associations are identified based on physical sedimentary and biological structures. The delta-plain facies association consists of weakly bioturbated mudstone, fissile mudstone, and coarse-pebbly stratified sandstone facies with sparse trace fossils. The delta-front–prodeltaic facies association contains muddy and sandy heteroliths, stratified, medium and coarse-grained, cross-bedded sandstone, and convoluted mudstone facies. The facies vary but characteristically contain trace-fossil assemblages of the recently proposed Rosselia and Phycosiphon Ichnofacies, indicating delta-front and prodeltaic settings, respectively. Variability in the facies is determined by the relative influence of hydrodynamic processes (tide, wave, and river), variations in physicochemical stress, and the episodic character of deposition. In addition, based on the relative influence of each hydrodynamic processes, the facies differ through the successions, sometimes subtly.

Microstructure and pore systems of shallow-buried fluvial mudstone caprocks in Zhanhua depression, east China inferred from SEM and MICP

Shallow-buried fluvial mudstones are of great significance as potential top seals for natural gas accumulations in the Zhanhua depression. Four samples were chosen to represent the range of fine-grained microfacies, from clay-rich to silt-rich to cemented, and mercury injection capillary pressure (MICP) porosimetry is combined with Scanning Electron Microscopy (SEM) to characterize microstructure and pore systems quantitatively. The nature of the pore systems that allow capillary breakthrough and thus leakage, were also estimated. Pore areas inferred from SEM data, obtained from representative elementary areas (REAs), follow a similar power law distribution to bulk sample MICP within a specific range, indicating that pores with areas larger than 103 nm2 are well connected. In samples without carbonate cement, pores within the clay matrix are larger in coarser-grained, siltier samples, and there are more pores at the edges of non-clay minerals; this results from force chains of large grains shouldering more effective stress. With increasing silt content, SEM-visible porosity increases and the contribution of pores between non-clay minerals grows significantly, while the contribution of pores within clay matrix reduces. In more clay-rich samples, capillary breakthrough is estimated to occur in pores associated with the clay matrix; in siltier samples, breakthrough will occur at lower entry pressures associated with larger interparticle pores. Carbonate cements play a key role in reducing pore space in some siltier samples by partially filling interparticle pores at (a) the interfaces between clay and non-clay minerals, and (b) pores between non-clay minerals areas larger than 106 nm2. By filling larger pores, capillary breakthrough in carbonate-cemented samples occurs at relative high entry pressures through pores in the clay matrix. However, carbonate cements, generally less than 20%, are not sufficient to enable silt-rich mudstones to become effective barriers. Clay content is the most critical control on mudstone seal capacity.

Characteristics of Mudstone in Complex Fluvial Sedimentary System in Bohai L Oilfield

Bohai L oilfield develops a complex fluvial sedimentary system, which includes many types of fluvial sedimentary facies. Based on the coring well of oil field, the distribution characteristics of mudstone are analyzed, it is shown that the mudstone has similar internal structure and can be classified into four types according to its color: gray to grayish, variegated, brown to gray-brown and khaki, the assemblage has a large gray to grayish mudstone section, large gray to grayish mudstone intercalated thin layer sandstone, gray to grayish mudstone associated with variegated (brown) mudstone, grayish mudstone associated with lacustrine sand grain bedding sandstone, concomitant generation of large staggered bedding sandstone and grayish mudstone, mixed (gray-brown) mudstone associated with large staggered bedding sandstone, interaction between different colors of mudstone and sandstone and large interlaced sandstone intercalated with thin layer mudstone. The mudstone color is mainly gray and grayish, and very few oxidized mudstone is developed alone, which indicates that fluvial mudstone may be formed in the reductive environment in humid climate, and the fluvial mudstone in this area may be formed in the oxidizing environment, which is different from the general understanding of fluvial facies, the oxidation color is the result of the later transformation.

Petroleum source rock potential of Whitehorse trough: a frontier basin in south-central Yukon

Abstract Whitehorse trough is a frontier basin in south-central Yukon that is thought to contain gas and possibly oil. It consists of up to 7000 m of sedimentary and volcanic rocks referred to as the Lewes River Group (Triassic), Laberge Group (Jurassic), and Tantalus Formation (Jura-Cretaceous). The Lewes River Group is informally subdivided into the Povoas and Aksala formations, and the Laberge Group is informally subdivided into the Richthofen, Nordenskiold and Tanglefoot formations. To evaluate the petroleum source-rock potential of this basin, over 600 samples from the Aksala, Richthofen, Tanglefoot and Tantalus formations from throughout the trough have been analyzed by Rock-Eval programmed pyrolysis and combustion and by study of thermal alteration indicies (TAI) of palynomorphs, conodont alteration indicies (CAI), and vitrinite reflectance. The Povoas and Nordenskiold formations were not sampled because they consist of volcanic and volcaniclastic rocks, respectively, and have no source rock potential. The analyses indicate that the Aksala formation is a poor source rock that is postmature with no hydrocarbon potential. The Richthofen formation is a poor source rock that is postmature with minor gas potential. The Tanglefoot formation is a good source rock that is immature to early mature and gas-prone. The Tantalus Formation is also a good source rock that is immature to early mature and gas-prone. The Tanglefoot and Tantalus formations contain petroleum fluid inclusions that were probably generated from within these units. The inclusions reveal homogenization temperatures of mostly between 75–115°C and reveal several distinct, but minor pulses of liquid petroleum (mainly 19–21°API, 23°–35°API and 40–46° API). Potential petroleum (gas) generative intervals occur at surface and in the shallow subsurface in deltaic mudstones (Tanglefoot formation) and fluvial mudstones (Tantalus Formation). The lateral extent of these intervals is unknown due to poor outcrop exposure and limited shallow drilling. Both the Tanglefoot and Tantalus formations are interpreted as potential source rocks and possibly effective source rocks. The most prospective areas for petroleum exploration are Five Finger Rapids, Division Mountain and Tantalus Butte and in the northern portion of Whitehorse trough.

Carboniferous biostratigraphy of the Rookhope Borehole, Co. Durham

AbstractA deep borehole drilled at Rookhope in Weardale, Co. Durham in 1960–61 proved the Weardale Granite below Carboniferous sediments at a depth of 390 m; an account of the Carboniferous rocks is presented. The boring commenced in the basal Namurian Great Limestone and entered the Dinantian at the bottom of the limestone at 25 m. Most of the ten Brigantian cyclothems sectioned in the borehole are compound and average 30 m thick. They consist of an initial cycle often more than 20 m thick and overlying minor cycles that are usuallyless than 6 m thick. Each of the cycles, both initial and minor, are coarsening upwards sequences from marine limestone or mudstohe to deltaic and fluvial mudstone, sandstone, seatearth and occasionally a thin coal seam. The underlying Asbian consists of seven cyclothems averaging 8 m thick. By contrast they are simple coarsening upwards sequences similar to the Brigantian initial cycles, but no minor cycles are developed.Marine fossils are abundant in the majority of cyclothems and are listed against their stratigraphical horizon in the borehole. Special studies of miospores, conodonts and foraminiferans from the borehole are presented. A consensus of biostratigraphical evidence shows that the base of the Carboniferous succession in the borehole is of early Asbian age.

Tectonostratigraphic development of the Interior Sudan rifts, Central Africa

In the Muglad, Melut and Blue Nile rift basins of Interior Sudan three major episodes of rifting, concomitant subsidence and nonmarine/nonvolcanic sedimentation are recognized. These three rifting cycles, which spanned 140 to 95 Ma (Fl), 95 to 65 Ma (F2), and 65 to 30 Ma (F3), resulted in the accumulation of up to 5400, 4200 and 5400 m of sediments, respectively. In the Muglad Basin, the best known and largest of the Sudan rift basins, each rifting cycle consists of (1) a basal sandstone unit (at least near rift margins), that is followed by (2) an upward coarsening section of lacustrine shale grading through marginal lacustrine mudstone and sandstone into fluvial mudstone and sandstone, and (3) a capping blanket of fluvial and alluvial sandstone. The shale-dominated portions of these cycles were deposited in a closed-drainage basin during active faulting. The fluvial and alluvial blanket sands were deposited in an open-drainage basin during the thermal sag phase following each tectonic cycle. The Early Cretaceous F1 intracontinental rifts of Interior Sudan were linked to major rifts/spreading centres in the Proto-South Atlantic by the dextral WSW-trending Central African Shear Zone and to the Indian Ocean via the NW-trending Anza rift in Kenya. In the Muglad Basin, F1 deformation involved high strain rates, rapid syn-rift crustal stretching and subsidence, and the formation of deep, fault-bounded tensional and transtensional pull-apart basins. During the F2 and F3 deformations, the rates of subsidence and stretching were much lower and were focused within smaller geographic areas. Structural elements include asymmetric half-grabens and less common full-grabens with central highs. The three superimposed tectonic episodes resulted in the subsidence of NNW- to NW-trending rift sub-basins; this gave rise to a wide variety of normal fault geometries, displacements, and growth histories. Planar domino-style and listric normal F1 fault arrays are modeled. The rotated F1 basement blocks typically are asymmetric and low-standing, and favour NE-directed growth and material transport. The F2 and F3 normal faults, which have both NE- and SW-directed polarities, often are listric and decouple younger syn-rift strata from older F1 rotated and locked planar basement faults. Palinspastic restorations and forward modeling of three regional cross-sections suggest that F1 faults account for 65–80% of total crustal extension. Stretching factors across the Interior Sudan rifts vary between 1.25 and 1.40. This range is considered to be a minimum since restored sections do not cross F3 depocenters. Total crustal extension across the Muglad plus the Melut rift basins exceeds 75 km.

Isotopic Imprint of Climate and Hydrogeochemistry on Terrestrial Strata of the Triassic-Jurassic Hartford and Fundy Rift Basins

ABSTRACT Late Triassic to Early Jurassic terrestrial sequences in the Hartford and Fundy rift basins have distinctive carbon and oxygen isotopic compositions of calcite and dolomite. The isotopic data mostly reflect paleoclimatic fluctuations and hydrogeochemistry of the lacustrine, playa, and fluvial environments. Dolomites from laminae in three sequences of playa red mudstones and lacustrine gray to black mudstones in the Hartford basin have variable isotopic compositions (13C = -5.8 to + 1.8 PDB; 18O = -7.2 to +0.7 PDB). Within any single symmetrical cycle of playa red mudstone--lacustrine gray, black, gray mudstone--playa red mudstone, there is a systematic change to relatively enriched 13C compositions in dolomite in the grayish black and black mudstones in the center of the cycle. These carbon isotopic data suggest that the lacustrine sequences formed as the lakes changed from well mixed with anoxic bottom waters to stratified with anoxic bottom waters where 13C-depleted carbon was concentrated in organic matter that was then buried. Calcites from lacustrine, micritic, and biomicritic limestones of the Scots Bay Formation of the Fundy basin have stable isotopic compositions (13C = -4.6 to -2.2 PDB; 18O = -6.1 to -3.0 PDB) that become more enriched in 18O and 13C upward in shallowing depositional sequences. These isotopic data reflect initial calcite precipitation when a high inflow of fresh water produced high lake levels, followed by progressively lower inflow, resulting in lower lake stands and higher salinity due to continuing evaporative loss of surface wat r. The lake waters were well oxygenated at all times. In the Hartford basin, caliche calcites in fluvial mudstones and sandstones have isotopic compositions (13C = -7.3 to -3.8 PDB; 18O = -8.0 to - 5.6 PDB) that reflect paleosol processes during climatic conditions that varied from warm and dry in Late Triassic time to relatively cooler and probably wetter in the Early Jurassic. Isotopic compositions of caliche calcites in redbeds in the Fundy basin indicate a parallel climate change from Late Triassic to Early Jurassic time, but also that the climate was relatively hotter and probably drier over the entir interval, as compared to the Hartford basin.

Stratigraphy of the Cretaceous-Tertiary and Paleocene-Eocene Transition Rocks of Big Bend National Park, Texas

The marine to terrestrial transition in the Big Bend area falls within the Late Cretaceous Aguja Formation, and, in light of new biostratigraphic data resulting from screening for small vertebrates and magneto-stratigraphic data, the Cretaceous-Tertiary boundary falls within the Javelina Formation, which includes the first red banding produced by oxidation of overbank fluvial mudstones. No record of a catastrophic event is apparent in the Javelina Formation. The Javelina, Black Peaks, and Hannold Hill Formations and the Big Yellow Sandstone Member of the Canoe Formation record increasing uplift in the region, culminating in uplift and volcanism in the Chisos Mountains, the source for upper Canoe Formation sediments. The sequence of changes produced by this trend and by unroofing in source highlands to the west is sufficiently gradual that the Javelina through Black Peaks units are not lithostratigraphically distinct at the formation level and therefore are reduced to member status, and placed, along with ...