Arthur H. Saller

Linked lowstand delta to basin-floor fan deposition, offshore Indonesia: An analog for deep-water reservoir systems

In offshore East Kalimantan, Indonesia, three-dimensional seismic reflectors can be traced downslope from a lowstand delta to a basin-floor fan, giving insight into depositional processes controlling the distribution of sands that serve as hydrocarbon reservoirs in many ancient deep-water settings. The studied interval includes the last three Pleistocene cycles (10–330 ka; each 110 k.y. in duration). Cycles on the shelf are dominated by progradational packages deposited during highstands and falling eustatic sea level. Progradational packages are separated by parallel reflectors and carbonate buildups of the transgressive systems tracts. During the last two lowstands of sea level (18 and 130 ka), coarse clastics were not deposited in deep-water environments because growth faults and regional subsidence prevented lowstand deltas from reaching the slope. During the lowstand of sea level that ended at about 240 ka, a delta prograded over the previous shelf edge, and sand-rich sediments spilled onto the slope.Strata on the slope and basin floor show how a deep-water depositional system evolved during a single cycle of eustatic sea level. A canyon on the slope connects the 240-ka lowstand delta to a coeval basin-floor fan. The canyon has a sinuous, bipartite fill that consists of a lower, amalgamated channel complex and an upper channel-levee complex. The basin-floor fan formed at the toe of the slope also has two parts. The stratigraphically lower part of the basin-floor fan has broad lobes with relatively continuous reflectors. The stratigraphically higher part has a sinuous channel-levee complex that prograded over the lower fan and fed sheetlike lobes on the outermost fan. The amalgamated channel fills on the slope and sheetlike lobes on the basin-floor fan have moderate- to high-amplitude reflectors and are inferred to represent sand-rich, early lowstand deposits. The channel-levee complexes on the slope and basin floor are dominated by low-amplitude reflectors and are inferred to be mud-rich strata deposited during the late lowstand. Unlike classic sequence-stratigraphic models, these lowstand strata do not onlap the slope; instead, deep-water clastics extend from the last clinoforms of lowstand deltas. In this system, lowstand deltas determined when and where sand-rich sediments entered preexisting canyons on the slope to feed basin-floor fans.

Distribution of Porosity and Permeability in Platform Dolomites: Insight from the Permian of West Texas

Saller and Henderson (1998) presented an article concerning porosity and permeability variations in two west Texas oil fields (North Riley and South Cowden fields). These fields occur within dolomitized Permian platform carbonates near the basinward shelf edge of the Central Basin platform, a paleogeographic element that influenced Permian deposition in west Texas. Their carefully documented study outlines several anomalous aspects concerning porosity distribution. They noted, in particular, that porosity distribution is not strictly facies dependent but that porosity tends to increase in a basinward direction. They suggested that porosity developed largely during dolomitization by the action of hypersaline evaporative fluids that flowed downdip in a basinward direction during the Permian. These brines were generated in restricted shelf interior lagoons and moved downdip parallel with bedding in a shallow reflux type of hydrologic system driven by subsurface fluid density differences in the Permian Clear Fork and Grayburg formations, as shown here in Figure 1 (see figure 18 in Saller and Henderson [1998]). Figure 1 Part A shows the scenario proposed by Saller and Henderson (1998) for porosity creation, dolomitization, and dolomite cementation of the Permian Clear Fork and Grayburg formations of west Texas. They suggested that macrodissolution of calcium carbonate occurred contemporaneously with dolomitization and dolomite cementation. This process of brine backreaction, however, would cause a net increase of porosity in the shelf interior, the opposite of what is observed. Part B shows a two-stage scenario proposed here to circumvent the problem of limestone macrodissolution by shelf interior brines that must have been initially in approximate bulk equilibrium with calcium carbonate. The first stage records dolomitization by replacement of limestone with little macrodissolution. In stage two, evaporative shelf interior brines may have evolved in equilibrium with dolomite only and became undersaturated with calcite and aragonite. Reflux migration of these brines downdip may …

Radiaxial calcite in lower Miocene strata, subsurface Enewetak Atoll

ABSTRACT Radiaxial calcite is abundant in Lower Miocene strata, 377-819 m deep on Enewetak Atoll. These radiaxial crystals have equant, bladed, and fibrous morphologies and occur as isopachous crusts 0.05-2.0 mm thick. Radiaxial terminations consist of fibrous subcrystals bundled together into larger acute, obtuse, or rounded shapes. Enewetak radiaxial calcite ranges from clear to very inclusion-rich. Most inclusions are microcavities caused by organic infestations (Videtich 1985). Alternating bands of inclusion-rich and inclusion-poor calcite mimic existing termination patterns. Radiaxial calcites fill intergranular pores, sponge borings, fractures, and aragonite molds. Aragonite has been pervasively dissolved from the Lower Miocene strata containing radiaxial calcite. Banded radiaxial calcit can be traced from intergranular pores into aragonite molds. Radiaxial calcites have a mean magnesium concentration of 3.2 mole % MgCO3 with a range from 1.6 to 11.1 mole % MgCO3. Inclusion-rich bands and more deeply buried radiaxial calcite tend to have lower magnesium concentrations. Enewetak radiaxial calcite has 13C values of +1.3 to +2.5 (PDB) and 18O values of -1.8 to +0.4 (PDB). Petrographic and stable isotopic data are consistent with precipitation of radiaxial calcite in its present form directly from seawater. Precipitation of radiaxial calcite after aragonite dissolution indicates precipitation distinctly after deposition, probably in a burial environment. Dissolution of aragonite by deep marine water is suggested by: 1) abundant marine cement associated with aragonite dissolution; 2) a lack of meteoric cements associated with aragonite dissolution; and 3) no evidence of subaerial exposure in Lower Miocene strata between 400 and 819 m. Modern Pacific Ocean water is undersaturated with respect to aragonite at depths where aragonite dissolution is observed (375-850 m). Tidal fluctuations and temperature profiles indicate that Lower Miocene strata containing radiaxial calcite are in open communication With modern Pacific Ocean water. Therefore, aragonite dissolution and radiaxial cementation are interpreted as having occurred in seawater (undersaturated with respect to aragonite) circulating through the atoll. Variations in magnesium concentration in Enewetak radiaxial calcite may be the result of original heterogeneity or differential loss of magnesium during later diagenesis in seawater. The first alternative is preferred. Precipitation of radiaxial calcite at different rates (at different degrees of saturation) could cause variable magnesium concentrations. More deeply buried radiaxial calcite may have precipitated more slowly in deeper, less supersaturated water, resulting in lower magnesium concentrations. Likewise, slower rates of precipitation may have resultedin organic infestations and lower magnesium concentrations in inclusion-rich bands. Enewetak radiaxial calcites suggest that two factors might be critical to development of radiaxial fabric: 1) precipitation at fluctuati g rates, and 2) formation in waters undersaturated with respect to aragonite.

Origin, Migration, and Mixing of Brines in the Permian Basin: Geochemical Evidence from the Eastern Central Basin Platform, Texas

Formation waters from producing reservoirs in Paleozoic carbonates have been studied to determine the origin of brines on the eastern Central Basin platform in west Texas. Chemical and isotopic analyses of these waters indicate mixing of brines of quite different origins in the deep subsurface of the Permian basin. Formation waters from the middle Permian San Andres Formation at 1430 m (4700 ft) and from Devonian limestones at 3200 m (10,500 ft) have salinities of 26-59 g/L and dD-d18O values in the same range as modern precipitation and groundwater in the near-surface Ogallala aquifer. Na-Cl-Br concentrations and molar ratios show that the salinity of these waters was largely acquired through halite dissolution. Formation waters from Pennsylvanian and Lower Permian shelf limestones at 2600-3000 m (8500-9800 ft) are more saline (70-215 g/L) and apparently represent a mixture of two different fluids. One end member was highly saline, derived from seawater evaporated well beyond halite saturation, and the other end member was a moderately saline meteoric water similar to the San Andres and Devonian formation waters. Halite beds occur only in Upper Permian (upper Guadalupian and Ochoan) strata in this area; hence, extremely saline evaporated seawater apparently descended into the Paleozoic carbonates during halite deposition in the Late Permian, mixing with and displacing marine formation waters by buoyancy-driven convective flow. These modified evaporitic brines were the dominant fluids in the Paleozoic carbonates until the late Tertiary, when meteoric water began to flow into deeper Paleozoic strata from outcrops and near-surface aquifers in southeastern New Mexico in association with tectonic uplift that began at 5-10 Ma. The meteoric water dissolved halite and anhydrite from Permian evaporites near the basin margin and moved eastward along the regional hydraulic gradient, mixing with and displacing the modified evaporitic brine in deeper hydrogeologic systems. These late Cenozoic meteoric fluids probably are responsible for widespread biodegradation of oil in the San Andres/Grayburg interval. The results of this study indicate that meteoric waters can migrate large distances and displace saline waters deep in a basin that has numerous oil reservoirs that have solution-gas drive. Understanding the history of formation waters can assist in exploration and production through improved (1) interpretation of reservoir-rock diagenesis, (2) prediction of oil biodegradation and displacement, (3) understanding of subsurface water pressures, and (4) interpretation of hydrocarbon saturations from resistivity logs.

Evolution and distribution of porosity associated with subaerial exposure in upper Paleozoic platform limestones, West Texas

Middle Pennsylvanian-Lower Permian limestones in the subsurface of west Texas were studied to determine how subaerial exposure and freshwater diagenesis (karstification) affected porosity distribution in meter-scale cycles. Approximately 87 depositional cycles are present in the gross reservoir interval (depths of 2600-3000 m), and each cycle is interpreted to represent a glacio-eustatic sea level fluctuation. Using recent radiometric age dating, average cycle duration is estimated at 160,000 yr per cycle. Reservoir-grade porosity (>4%) occurs in 5-25% of the gross reservoir section. Porosity is stratified, occurring in 1-6-m-thick intervals in the upper part of cycles that were subaerially exposed; however, many cycles that were subaerially exposed now lack porosity. Diagenesis and porosity development have distinct patterns related to duration of subaerial exposure. Four stages of porosity development are identified. (1) Very brief or no subaerial exposure (estimated at less than 5000 yr) caused little or no diagenetic change. (2) Brief to moderate subaerial exposure (estimated at 5000-50,000 yr) resulted in most primary pores being filled with calcite cement, and dissolution creating fine matrix pores (molds and intercrystalline pores). (3) Moderately long subaerial exposure (estimated at 50,000-130,000 yr) resulted in cements filling primary pores and some fine secondary pores, and dissolution creating small conduit pores (vugs, fractures, fissures). (4) Prolonged subaerial exposure (estimated at greater than 130,000 yr) resulted in most primary and secondary matrix pores being filled with calcite cement, but dissolution enlarged conduit pores (vugs, fractures, fissures). Present subsurface porosity in this field preferentially occurs in thick grainstones, phylloid algal boundstones, and a few wackestone/packstones in cycles subjected to brief subaerial exposure (stages 2 and 3). Matrix porosity (molds, intercrystalline pores) is dominant because most conduit pores formed during prolonged subaerial exposure were filled with either shale during subsequent transgressions or burial cements derived from pressure solution associated with the shales. The distribution of porosity in the Southwest Andrews area indicates that duration of subaerial exposure and supply of clastics are major factors determining ultimate porosity in limestones subjected to subaerial exposure and karstification.

Cycle stratigraphy and porosity in Pennsylvanian and Lower Permian shelf limestones, eastern Central Basin Platform, Texas

Pennsylvanian and Lower Permian shelfal limestones were studied in core and wireline logs on the eastern side of the Central Basin platform in west Texas. Sixty-three (63) cycles were delineated in the study interval, which includes 200250 m of Canyon (Missourian), Cisco (Virgilian), and Wolfcamp strata. Four general lithofacies are present: fossiliferous wackestones and packstones, grainstones, phylloid algal boundstones, and shales. These lithologies typically occur in 118-m-thick cycles bounded by subaerial exposure surfaces. Grainstones in the upper part of some cycles indicate a shallowing of environments prior to subaerial exposure. Many cycles have subaerial exposure surfaces developed on subtidal fossiliferous wackestones or packstones suggesting rapid falls in se level. Long-term transgressive intervals (transgressive systems tracts or TST) are dominated by thick (>4 m) cycles, whereas long-term regressive intervals (highstand systems tracts or HST) are dominated by thinner cycles. Stable carbon isotope data suggest that thick cycles in TSTs were subjected to short periods of subaerial exposure, whereas thin cycles in the HSTs were subjected to much longer subaerial exposure. Porosity is quite variable beneath subaerial exposure surfaces. At any given well, reservoir-grade porosity (>4%) is present below only 1045% of the identified subaerial exposure surfaces. Where pres-ent, reservoir-grade porosity occurs in the upper part of cycles, 0.35 m below subaerial exposure surfaces. Porosity in thick cycles of the TST is relatively widespread and not facies-selective. Porosity is less abundant and facies-selective in the HST, with porosity occurring only in relatively thick (1.54 m) grainstones, which are concentrated near the shelf margin. Thin (

Presalt stratigraphy and depositional systems in the Kwanza Basin, offshore Angola

The Lower Cretaceous presalt section in the Kwanza Basin contains an excellent petroleum system that includes “synrift” strata (Barremian) overlain by a “sag” interval (Aptian) and capped by the Loeme Salt. The upper synrift is generally limestone with widespread mollusk packstones and grainstones (coquinas) deposited in a fresh-to–moderately saline (alkaline) lake. The sag interval is characterized by carbonate platforms and silica-rich isolated buildups formed in highly evaporated, highly alkaline lakes. Shrubby (dendritic), microbially influenced boundstones and intraclast–spherulite grainstones accumulated in shallow water on platform tops. Microbial cherts were deposited as organic buildups on large, isolated structural highs basinward (west) of platforms, and they apparently formed at low temperatures in very alkaline lake water. Shrubby boundstones and microbial cherts have vuggy pores that are primary and result in high permeability. Wackestones and packstones with calcitic grains (mainly spherulites) in dolomite or argillaceous dolomite were deposited in slightly deeper, low-energy sag environments. In addition, clays (especially stevensite) precipitated out of the silica-rich, highly alkaline lake waters. During sag deposition, calcite precipitated on the shallow lake floor with morphologies that ranged from spherulites to shrubs and included a continuum of intermediate forms. Spherulites probably precipitated just below the sediment–water interface. Spherulites and shrubby calcites are commonly recrystallized. Spherulites floating in stevensite probably formed in deeper lacustrine environments. Organic-rich mudstones were deposited in even deeper lacustrine environments in synrift and sag intervals, and they are likely the source of most hydrocarbons in this system. These interpretations are supported by seismic, core, petrographic, and stable isotope data.

Relationship of Uranium to Petrography of Caliche Paleosols with Application to Precisely Dating the Time of Sedimentation

ABSTRACT We use petrography combined with fission-track mapping to evaluate qualitatively the phases that concentrate uranium in caliche paleosols from late Paleozoic cyclothems of Texas and New Mexico. We also discuss geochemical analyses of U and Pb concentrations and Pb isotopic compositions. Uranium concentrations in brown caliche calcite range from 1 to 9 ppm. Lead concentrations in the same calcites range from 0.5 to 1 ppm. Lead-isotope compositions range from common (no apparent radiogenic component) to lead with a substantial radiogenic component. Caliches with brown calcite most likely developed near the vadose-phreatic interface--often a redox boundary where organic material (and uranium) is expected to be concentrated. In contrast, uranium concentrations in light-colored caliche nodules from the vadose zone have uranium concentrations that are 1 ppm or less. Fifteen of forty-five samples from this study have a range in U/Pb that would permit precise dating of the rocks, but only the data from three give precise ages. Some of the samples have hematite, which has high concentrations of U and Pb. For one of the precisely dated samples, the two-sigma uncertainty was improved from 10 to 2.6 Ma by avoiding hematite when sampling.

Reservoir characteristics of Devonian cherts and their control on oil recovery: Dollarhide field, west Texas

Approximately 70 million bbl of oil have been produced from the chert-dominated Thirtyone Formation at Dollarhide field. The Thirtyone Formation is Devonian in age and contains two reservoir intervals, an upper dolomite and a lower chert, separated by nonporous limestone. The chert reservoir contains approximately 83% of the original oil in place (OOIP) and consists of two different facies--laminated microporous chert and burrowed chert. The laminated microporous chert was apparently deposited as sponge spicule sands (grainstones) in channels and fans on the slope of the Tobosa basin. The burrowed chert facies was apparently deposited as burrowed mixtures of sponge spicules, siliceous mud, and carbonate mud in broad slope environments between fans and channels. Spicules are thought to be derived from disaggregation of siliceous sponges living on the slope. Early marine and/or meteoric diagenesis dissolved many sponge spicules and reprecipitated the silica as microcrystalline chalcedony and quartz, resulting in spicule-moldic porosity and microporosity in the originally siliceous strata. The burrowed chert underwent small-scale differential compaction that produced short, discontinuous fractures, 1-10 mm in length and 0.1-0.5 mm in width. Production from the field has been related to amounts and characteristics of the pore systems. The thickness of the porous chert ranges laterally from 0 to 80 ft (0-24 m), and pore volume (fh) ranges from 0 to 20 pore ft (0-6 pore m). The laminated microporous chert is very homogeneous and has high porosity (25-35%) and uniform permeability (5-30 md). Areas dominated by laminated microporous chert had moderate primary recovery (approximately 200,000 bbl of oil per well), excellent waterflood production (1-2.5 (Begin page 36) million bbl of oil per well), and poor 20-ac (8.1 ha) infill production (<20,000 bbl of oil per well). The burrowed chert has more heterogeneous porosity (5-30%) and permeability (<1-100 md). The dolomite also has heterogeneous pore networks that have porosities of 3-15% and measured permeabilities of less than 1-200 md. Wells having substantial amounts of porous dolomite and/or burrowed chert had moderate to good primary recovery (200,000-300,000 bbl of oil per well), moderate waterflood recovery (300,000-1,100,000 bbl of oil per well), and moderate 20-ac (8.1 ha) infill recovery (50,000-100,000 bbl of oil per well). Oil recovery from the CO2 flood is expected to average approximately 250,000 bbl per well, having slightly higher recovery (300,000 bbl) in areas that have thick laminated microporous chert and/or burrowed chert. Oil recovery from the homogeneous, laminated microporous chert during the CO2 flood is improved by large amounts of residual oil and good sweep efficiency but decreased by the previously efficient sweeping of original mobile oil by the waterflood. The CO2 recovery from the heterogeneous, burrowed chert is improved by significant amounts of unswept mobile oil but decreased by poorer sweep efficiency. As a field, primary recovery is estimated at 13% of OOIP, waterflood at 30% of OOIP, infill at 3.5% of OOIP, and CO2 flood at 11% of OOIP.

Alluvial to Marine Facies Transition in the Antler Overlap Sequence, Pennsylvanian and Permian of North-Central Nevada

Autochthonous sedimentary rocks of the Antler overlap sequence indicate regional subsidence of north-central Nevada during Pennsylvanian and Early Permian time as an inland sea gradually transgressed over coarse alluvium eroded from remnants of the Antler orogenic highlands. Alluvial conglomerates and sandstones of the Lower Atokan (Middle Pennsylvanian) Battle Formation (up to 240 m thick) rest unconformably on folded and faulted lower Paleozoic strata that were deformed by emplacement of the Roberts Mountains allochthon during the Antler orogeny. Paleocurrents and stratigraphic relations suggest that the Battle Formation represents deposits of several paleotributaries of a single major paleovalley draining to the southwest. The Battle Formation and overlying limestones constitute a conformable transgressive sequence. In ascending order, the lithofacies and corresponding depositional environments of the Battle Formation are: 1) massive clast-supported conglomerate deposited in proximal portions of a braided-stream system, 2) very sandy conglomerate with rare foreset cross-stratification deposited in the middle reaches of a braided-stream system, 3) cross-stratified conglomeratic sandstone deposited in the distal part of a braided-stream system, 4) fine conglomerate and fine-grained, commonly muddy, sandstone deposited near the terminus of a braided-stream system, and 5) interbedded calcareous sandstone, calcareous conglomerate, and mudstone of tidal and deltaic origin. The Battle Formation is overlain by Pennsylvanian carbonates (Etchart Formation, Highway Limestone, and Antler Peak Limestone) deposited in shallow-marine and/or marginal-marine environments. Paleontologic evidence indicates roughly contemporaneous deposition of 1) open-marine carbonates within a shallow inland sea, 2) mixed terrigenous-carbonate sediments around the margins of the shallow sea, and 3) conglomerates and sandstones in alluvial environments farther inland.