Sisquoc Formation Research Articles

Case Study: Steam-Injection Step-Rate Tests Run in the Orcutt Oil Field

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 169513, “Case Study: Steam-Injection Step-Rate Tests Run in the Shallow Low-Permeability Diatomite Formation, Orcutt Oil Field, Careaga Lease, Santa Barbara County, California,” by Ramon Elias and Ian Marquardt, Santa Maria Energy, and Mason Medizade, California Polytechnic State University, prepared for the 2014 SPE Western North American and Rocky Mountain Joint Regional Meeting, Denver, 16–18 April. The paper has not been peer reviewed. The operator has initiated a cyclic-steam- stimulation (CSS) project in the Opal A diatomite of the Sisquoc formation on the Careaga lease in the Orcutt oil field in Santa Barbara County, California. The operator has received entitlement to proceed with an expansion consisting of 110 additional new wells. This paper discusses steam-injection step-rate tests (SRTs) for this asset. Introduction The target zone contains high oil content ranging from 1,800 to 3,000 bbl/acre-ft in massive intervals with 200- to 700-ft thickness at depths of 600 to 1,000 ft and with a permeability of 5 to 15 md. The pilot currently consists of 19 cyclic-steam-injection wells configured in a 4×5 matrix spaced approximately 120 ft apart, producing from an average depth of 925 ft. A supervisory control and data-acquisition system is used to control and monitor various aspects of field operations, especially the steam- injection-process protocols. Relatively low steam-injection rates of 250 to 450 B/D coldwater equivalent are used with 70% constant- quality steam. Steam-injection pressures and rates have been monitored closely throughout the process since startup in October 2009. More than 1,000 steam-injection cycles have been completed as of this paper’s writing. For all cycles, the onset of formation parting has been detected less than 0.5% of the time. In all detected events, steam injection was adjusted or halted without loss of steam-injection containment. Steam injection was resumed when it was determined to be safe to do so, and matrix flow was re- established. To this end, two operational procedures exist for analyzing data as part of the operator’s efforts to improve steam-injection practices further and to improve understanding of the mechanisms leading to oil production. One is taken from common waterflood surveillance practices and graphically displays changes in flow resistance during injection, aimed at indicating if formation parting or a loss of steam confinement is initiating. The other is analysis of the pressure falloff observed during the soak period between injection and production, a period intended to allow pressures and temperatures to dissipate away from the vicinity of the well and into the producing zone. General SRT Approach The objective of an SRT is to determine the pressure at which resistance to flow within the rock matrix suddenly decreases as the formation is subjected to increasingly higher fluid-injection rates. That point is said to be detectable when the injection pressure exceeds formation-parting pressure (FPP), which creates increased fluid conduction. This effect produces a reduction in the slope of the pressure-vs.-flow-rate curve. The values of pressure and flow rate corresponding to the point where the two lines intersect in the pressure-vs.-flow-rate curve are called the FPP and formation-parting rate (FPR), respectively (see Fig. 1 in the complete paper). The FPR can be unique to the injection fluid that is used. In general, an SRT consists of a series of constant-rate injections with rates increasing from low to high in a step-wise fashion following a period of reservoir stabilization. Each constant-rate step is normally run over approximately equal lengths of time. A discussion of SRT analysis by use of Hall’s method is provided in the complete paper.

Archaeal lipids in Neogene dolomites (Monterey and Sisquoc Formations, California) – Planktic versus benthic archaeal sources

The distribution of archaeal lipids, including archaeol and glycerol dibiphytanyl glycerol tetraethers (GDGTs), in dolomite concretions and surrounding sediment from the Monterey Formation (Miocene) and the Sisquoc Formation (Miocene–Pliocene) were examined to distinguish planktic from benthic contributions. For this purpose, dolomites with positive δ 13C values (+7‰ to +13‰) were chosen; such highly positive values point to pronounced methanogenesis of benthic archaea in the sedimentary column. At first glance, distributions and relative abundances of GDGTs in both dolomites and background sediment were similar, resembling patterns of marine planktic crenarchaea. A contribution of benthic euryarchaea to the GDGT pool became evident only from variations in the δ 13C values of different biphytanes obtained after ether cleavage of GDGTs. Whereas bi- and tricyclic biphytanes had an isotopic signal typical of planktic archaea (δ 13C −23.6‰ to −20.5‰ and −23.4‰ to −21.2‰, respectively) for both dolomite and background sediment, acyclic and monocyclic biphytanes showed lower values for dolomite samples (−25.1‰ to −22.6‰ and −27.6‰ to −24.7‰, respectively), indicating a contribution of lipids from benthic archaea. The isoprenoid diether archaeol (δ 13C −23.9‰ to −22.9‰), assigned to euryarchaea, was only detected in dolomite samples, also reflecting additional input from sedimentary archaea, probably autotrophic methanogens. The occurrence of lipids derived from methanogenic archaea agrees with the strong 13C-enrichment of dolomites and with mineral formation taking place in the zone of archaeal methanogenesis. This implies that the lipid biomarker inventory of sedimentary strata needs to be interpreted carefully, as it is often not straightforward to discriminate between input from the water column and sedimentary microbial activity.

Alteration and fracturing of siliceous mudstone during in situ combustion, Orcutt field, California

Changes in rock mineralogical composition and in fracture density and distribution resulting from natural in situ combustion of hydrocarbons were characterized to infer comparable processes of alteration and fracturing during enhanced oil production from heavy oil reservoirs by in situ combustion or fireflooding. Natural combustion alteration was studied in siliceous mudstone of the Miocene Sisquoc Formation at Orcutt oil field, California, where centers of most intense combustion alteration are composed of 1–2 m thick tabular zones of brecciated clinker. These centers are surrounded by 10–20 m wide alteration haloes of oxidized and sintered oxidized mudstone and an outer fringe of coked organic matter. Based on the stability of mineral phases around an individual combustion center, peak temperatures of combustion were estimated to have reached 1100 °C at the center of combustion, tapering off to about 350 °C at the outer edge of the coked zone. Changes in fracture density, distribution, and style were quantified based on fracture scanline measurements across alteration zones and in unaltered mudstone. With increasing alteration, newly formed fractures connect with and intersect preexisting tectonic joints, providing an isotropic permeability structure for fluid flow. Addition of newly formed fractures to the existing joint systems is distinctly developed in oxidized mudstone, corresponding to alteration temperatures of about 750–800 °C, and well developed in sintered oxidized mudstone that formed at inferred temperatures of about 900 °C. Fractures with large aperture to length ratios in clinker are inferred to have formed at peak temperatures of about 1100 °C. Based on alteration haloes around tectonic and combustion-induced fractures, it is demonstrated that these fractures contributed significantly to flow of air or steam during combustion. Combustion zone centers are inferred to follow faults and joint zones that contained hydrocarbons that migrated into these migration conduits prior to and possibly during combustion. The natural combustion alteration is interpreted as the result of slowly outward moving alteration fronts around stationary combustion centers. The observed alteration distribution and associated pattern of induced fractures may thus be considered a natural outcrop analog of alteration associated with a well-developed combustion front during fireflooding of heavy oil reservoirs. Although peak temperatures at Orcutt oil field likely exceeded temperatures characteristic of firefloods, fractures similar to those formed in the outer alteration zones may enhance the flow of oxidant to combustion fronts and of light hydrocarbons to production wells in firefloods.

Relationship of clathrate instability to sediment deformation in the upper Neogene of California

Widespread soft-sediment deformation (SSD) in upper-bathyal continental-margin sediments has been explained by compaction-related dewatering and related physical processes that can destabilize margin sediments. We present the hypothesis that SSD in such facies resulted in part from instability of gas hydrates (clathrates), leading to overpressurization in sediments and resulting fluid flow and brecciation. Clathrate-related deformation must be widespread in upper-bathyal continental-margin methanogenic sequences at depths where clathrates are destabilized by changes in bottom-water temperatures and sea level. A close association exists between an array of SSD features with clathrate-hosting sediments in ocean drill cores. Similar features are conspicuous in the partly laminated, upper Neogene upper bathyal sediments of the Sisquoc Formation in southern California, suggesting that clathrates may have been responsible for abundant and well-developed SSD in these units. The lithology, methane-producing organic content, and depositional environment of the Sisquoc Formation are similar to those features in modern clathrate-hosting settings. These characteristics, in conjunction with the upper bathyal paleobathymetry of the unit, the geothermal gradient of the region, and δ 13 C values of benthic foraminifera and dolomite concretions suggest the occurrence of clathrates in the shallow subsurface of these sediments. We suggest that the analyzed parts of the Sisquoc Formation represent an exhumed zone of SSD resulting from clathrate instability in the shallow subsurface of the continental margin. Sedimentary rocks exhibiting an association of SSD features that resulted from clathrate formation and dissociation are named clathrites.

Timing and Geometry of Left-Slip Faulting and Compressional Folding in Hondo Field, Western Santa Barbara Channel: ABSTRACT

Hondo field is located within the Santa Ynez offshore lease unit, approximately 5 mi south of Gaviota, California. Production is from Monterey fractured shales and deeper sandstone horizons. Two types of structures are well imaged on 3-D reflection data from the east end of the field, which has undergone 3-D prestack time migration. The most conspicuous structure is an east-west compressional fold with a gently to moderately dipping southern limb and a shorter and more steeply dipping north limb. The shape of the fold suggests that uplift was related to southward transport up a north dipping thrust ramp at (subseismic) depth. The northern limb of this structure is widest at the level of the top Sisquoc Formation and narrowest in sediments 2-3 cycles beneath the sea floor. This triangular panel of north dip indicates growth of the structure commenced during deposition of the lower Pico Formation in the Pliocene, and continued through the Pleistocene. The second-order structures that control trap distribution within the field include high-angle faults that trend roughly eastwest. The northernmost of these faults, the north bounding fault (NBF) is known from well data to have 1500 feet of throw and to dip steeply to the south. Itmore » would restore to a vertical or steep north dip upon removal of the effects of Pliocene-Pleictocene folding. The southernmost of these faults (SF) appears to change displacement from down-to-the-south in the east to down-to-the-north in the west and also dips steeply. A high-angle fault in the center of the field (CF) also has a variable sense of offset across it. Several prominent northeast-southwest faults lie between the CF and SF and clearly exhibit normal separation on vertical time sections and left separation on time slices. Variable thicknesses of Sisquoc Formation and roughly uniform thicknesses on Monterey Formation indicate that these faults were active in latest Miocene to early Pliocene.« less

Geochemistry of inorganic and organic sulphur in organic-rich sediments from the Peru Margin

We have determined (1) the abundance and isotopic composition of pyrite, monosulphide, elemental sulphur, organically bound sulphur, and dissolved sulphide; (2) the partition of ferric and ferrous iron; (3) the organic carbon contents of sediments recovered at two sites drilled on the Peru Margin during Leg 112 of the Ocean Drilling Program. Sediments at both sites are characterised by high levels of organically bound sulphur (OBS). OBS comprises up to 50% of total sedimentary sulphur and up to 1% of bulk sediment. The weight ratio of S to C in organic matter varies from 0.03 to 0.15 (mean = 0.10). Such ratios are like those measured in lithologically similar, but more deeply buried petroleum source rocks of the Monterey and Sisquoc formations in California. The sulphur content of organic matter is not limited by the availability of porewater sulphide. Isotopic data suggest that sulphur is incorporated into organic matter within a metre of the sediment surface, at least partly by reaction with polysulphides.Most inorganic Sulphur occurs as pyrite. Pyrite formation occurred within surface sediments and was limited by the availability of reactive iron. But despite highly reducing sulphidic conditions, only 35–65% of the total iron was converted to sulphide; 10–30% of the total iron still occurs as Fe (III). In surface sediments, the isotopic composition of pyrite is similar to that of both iron monosulphide and dissolved sulphide. Either pyrite, like monosulphide, formed by direct reaction between dissolved sulphide and detrital iron, and/or the Sulphur species responsible for converting FeS to FeS2 is isotopically similar to dissolved sulphide. Likely stoichiometries for the reaction between ferric iron and excess sulphide imply a maximum resulting FeS2 :FeS ratio of 1:1. Where pyrite dominates the pool of iron sulphides, at least some pyrite must have formed by reaction between monosulphide and elemental sulphur and/or polysulphide. Elemental sulphur (S0) is most abundant in surface sediments and probably formed by oxidation of sulphide diffusing across the sediment-water interface. In surface sediments, S0 is isotopically heavier than dissolved sulphide, FeS and FeS2 and is unlikely to have been involved in the conversion of FeS to FeS2. Polysulphides are thus implicated as the link between FeS and FeS2.

Origin and Diagenesis of Clay Minerals in the Monterey Formation, Santa Maria Basin Area, California

AbstractThe clay mineralogy of the Miocene Monterey Formation was determined for onshore and offshore sequences in the Santa Maria basin area, California. The <0.1-μm fraction of clayey, opal-CT porcelanites, siliceous mudstones, and dolostones from the Pt. Pedernales area consists of mixed-layered illite/smectite (I/S) that contains < 10% illite layers. The underlying Tranquillon Volcanics and Obispo Tuff, the presence of bentonite beds, zeolite minerals, and unaltered volcanic ash beds in the Monterey Formation, and the highly smectitic composition of the I/S suggest that a significant amount of the I/S formed from the alteration of vitreous volcanic ash. The alteration of volcanic glass to smectite coincides with the transformation of biogenic opal-A (mostly diatoms) to opal-CT. Alteration of volcanic glass to smectite may inhibit the opal-A to opal-CT transformation by removing pore-water Mg, promote dolomite precipitation by raising the pH, alter the Sr isotopic composition of the pore waters, and provide an additional source of silica to these predominantly biogenic siliceous rocks. The 0.1- to 2-μm fraction contains mica minerals (discrete illite, biotite, and muscovite) as well as I/S. Clayey, opal-A diatomites from the overlying Sisquoc Formation contain detrital kaolinite and random (R = 0) I/S, but the percent illite layers in the I/S is uncertain.The <0.1-μn fraction of quartz rocks from the Lions Head area and the offshore B-2 well is far more variable, and contains random or ordered (R = 1) I/S, kaolinite, and chlorite. Corrensite was observed at the base of the Lions Head area. The percent illite layers in the I/S increases from 10% to 80% over a stratigraphic depth of 0.8 km that corresponds to a present-day temperature range of 80 to 115°C. Initial illitization of smectite coincides with the opal-CT to quartz transformation. Ordered (R = 1) I/S is associated with abundant diagenetic kaolinite and dolomite in several metabentonite beds. The dominance of sodic plagioclase over K-feldspar, and the absence of kaolinite and chlorite in rocks of equivalent age that have not undergone illitization suggest that limited availability of K results in the alteration of smectite to kaolinite, chlorite, and possibly late dolomite. Minor amounts of clinoptilolite were found in opal-A and opal-CT rocks. Minor to trace amounts of analcime and mordenite were tentatively identified, primarily in quartz rocks.

Porosity reduction and burial history of siliceous rocks from the Monterey and Sisquoc Formations, Point Pedernales area, California

Porosity, grain density, and composition were determined for different rock types from a 1.2-km-thick Neogene section of the Monterey and Sisquoc Formations in the Point Pedernales area, California. Porosity decreases and dry bulk density increases with burial depth in these predominantly siliceous rocks. Porosity reduction is controlled largely by initial composition and silica diagenesis. There is a positive correlation between porosity and biogenic silica (diatom) content of diatomaceous shales. The porous, incompressible diatom frustule maintains a high porosity relative to detrital, organic, and calcareous components of the rock. Porosity reduction by physical compaction from increasing overburden pressure is much slower at burial depths greater than 0.5 km. Solution of the porous diatom frustule and precipitation of cryptocrystalline opal-CT resulted in a highly variable porosity reduction. Porosity reduction during the opal-A to opal-CT transformation depends on the amount, composition, and texture of the nonsilica grains. A large porosity reduction occurred locally in nodules and beds of pore-filling dolomite and silica. The porosity-depth relation of siliceous sediments and rocks from this and previous studies is used with biostratigraphic age data and estimated paleobathymetry to reconstruct the burial history of the Point Pedernales area. The sedimentation rate, corrected for compaction, quadrupled during deposition of the Sisquoc Formation. Using the constant water depth proposed for the Monterey Formation, and assuming that basin filling occurred during rapid deposition of the Sisquoc Formation, results in a tectonic subsidence curve similar to that observed for cooling oceanic lithosphere. Upwelling of asthenosphere near the surface would have produced a significantly higher paleogeothermal gradient, and the opal-A to opal-CT transformation is predicted to have occurred 3 m.y. earlier than if a constant, presentday geothermal gradient is assumed. Basin subsidence by faulting or flexure, without upwelling of asthenosphere, does not significantly alter the thermal history.

Geothermometry from Smectite and Silica Diagenesis in the Diatomaceous Monterey and Sisquoc Formations, Santa Maria Basin, California: ABSTRACT

Geothermometry from Smectite and Silica Diagenesis in the Diatomaceous Monterey and Sisquoc Formations, Santa Maria Basin, California: ABSTRACT

Stratigraphic Sequence of the Monterey and Sisquoc Formations in the Hondo Oil Field, Santa Barbara Channel, California: ABSTRACT

Stratigraphic Sequence of the Monterey and Sisquoc Formations in the Hondo Oil Field, Santa Barbara Channel, California: ABSTRACT

Gravity study of the boundary between the western Transverse Ranges and Santa Maria Basin, California

Three gravity profiles were completed across the inferred trace of the Santa Ynez River fault in southern California. This fault is believed to be the structural boundary between the Santa Maria Basin and the western Transverse Ranges to the south. A large, steep gravity gradient is associated with the inferred east‐west trace of this fault. Gravity to the north of the fault is approximately 60 mGal lower. The gravity anomalies were analyzed to answer two questions: What density contrasts (and therefore rock types) are juxtaposed across the fault at depth? What is the geometry of the fault at depth? Forward and inverse modeling was performed on the anomalies using structural constraints from geologic cross sections and a seismic refraction study plus density constraints from well logs. The Bouguer gravity was modeled as a combination of three sources: a deep long‐wavelength contribution from the slight northward deepening of the Moho; an upper crust contribution from the density contrast between the low‐density Miocene Monterey and Sisquoc formations (present in the Santa Maria Basin) and the denser shallow basement beneath the western Transverse Ranges to the south; and a high‐density midcrust contribution, extending from 3 to 11 km depth below the Transverse Ranges. The origin of the midcrust contribution is unknown, but we suggest that it may arise from a layer of oceanic crust beneath the western Transverse Ranges south of the Santa Ynez River fault.

Phosphatic Hardgrounds and Hiatus Concretions in Neogene Marine Sequences of California Coastal Ranges: ABSTRACT

Distinctive phosphatic intervals mark hiatuses of varying durations at several localities along the central California coast. At Capitola in the La Honda basin, a complex series of carbonate-cemented hardgrounds containing authigenic phosphatic nodules and marine vertebrate bones occurs within 50 cm of the upper Purisima Formation (Pliocene). Paleomagnetic measurements and diatom stratigraphy indicate that this 50-cm interval represents a hiatus of about 1 m.y. (4.5-3.5 Ma). At Naples Beach in the Santa Barbara basin, a severely condensed section of about 7 m of interbedded nodular phosphatic hardgrounds, phosphatic marlstones, and dolomites appears to represent as much as 5 m.y. (14.5-9.5 Ma). In the Lompoc Quarry of the Santa Maria basin, a thin phosphatic hardground at the contact between the Monterey and the Sisquoc Formations represents about 0.5-0.7 m.y. (between 7.0 and 6.0 Ma). At Mussel Rock in the Santa Maria basin, a nodular phosphatic conglomerate occurs at the boundary between the Monterey and Sisquoc Formations; the age of the conglomerate determined from diatom biostratigraphy is approximately 6.2 Ma (Messinian), but no time gap can be recognized given the resolution of diatom age dating. The phosphatic hardgrounds of Capitola, Lompoc Quarry, and Naples Beach appear to correspond to high sea levelmore » stands, whereas the conglomerate at Mussel Rock coincides with a sea level lowstand. During high sea level stands, depocenters shifted landward, leading to clastic-sediment starvation possibly accompanied by current-induced erosion and winnowing, which in term led to condensation and early diagenetic carbonate and phosphatic cementation.« less

Accumulation Rates of Biogenous Silica and Terrigenous Detritus in Late Miocene-Early Pliocene Santa Maria Basin, California: ABSTRACT

The upper-Miocene-lower Pliocene laminated to massive diatomaceous strata of the Santa Maria basin encompass portions of the Monterey and overlying conformable to nonconformable Sisquoc Formations and record the continued accumulation of admixed siliceous (mostly diatoms) and terrigenous (silt and clay) components in a Neogene borderland basin. Lithologies, which vary from diatomite to diatomaceous mudstone, are the result of fluctuations in slope-accumulating silica and terrigenous debris. Bulk accumulation rates, as well as silica and terrigenous component accumulation rates, were calculated for four measured sections using accurate stratal thicknesses, absolute ages, rock compositions, and bulk densities. Bulk accumulation rates range from less than 20 to greater than 70 mg/cm/sup 2//year and are comparable to bulk accumulation rates in the present-day Santa Barbara basin and Gulf o f California. Silica accumulation rates (20-30 mg/cm/sup 2//year) suggest that sedimentation took place beneath productive waters and they exceed those encountered in other high-productivity oceanic areas, such as the Bering Sea and Peruvian coastal waters. Relatively high terrigenous accumulation rates (18-35 mg/cm/sup 2//year) reflect the dilution of these high-productivity waters by continentally derived detritus. Comparison of calculated accumulation rates with rates determined for similarly aged strata of the Santa Barbara basin reveals that silica and terrigenousmore » debris were accumulating at higher rates within the Santa Maria basin. These differences are due to the complex interplay of tectonics and oceanographics and to the proximity of the basins relative to the strandline.« less

The generation of magnetic anomalies by combustion metamorphism of sedimentary rock, and its significance to hydrocarbon exploration

Low-temperature exothermic oxidation of fine pyrite in carbonaceous marine rocks can lead to spontaneous combustion. As a result, the organic material in the rocks is consumed as fuel, but unlike burnt coal seams and lignite beds, the host rock remains after combustion. Thermal metamorphism, and in some cases partial to complete melting of the rock, will result. Zones of combustion metamorphism are frequently associated with oil fields. These occurrences can be related to either surface outcrops of source rock or leakage from reservoirs. A study of the magnetic properties of a suite of combustion-metamorphosed specimens of the Monterey and Sisquoc Formations in southern California shows orders of magnitude increases in magnetic intensity and susceptibility for both thermally metamorphosed and fused samples. These changes reflect both the production of new magnetic minerals, derived mainly from oxidized iron sulfide, and their acquisition of thermal remanence. Ground magnetic surveys across combustion metamorphism zones in three southern California oil fields reveal anomalies of up to 2,000 gammas associated with the metamorphosed and melted rock. Large-scale aero-magnetic anomalies are also associated with extensive combustion metamorphism zones in the Marcelina Formation of Venezuela and the Ghareb and Taquiya Formations (the “Mottled Zone”) in Israel. The recognition of magnetic anomalies caused by combustion metamorphism in areas of oil production suggests that closely spaced low-altitude aeromagnetic surveying can be a useful petroleum-exploration tool.

Sedimentary Structures and Depositional Environments of Diatomaceous Strata of Sisquoc Formation, Santa Maria Area, California: ABSTRACT

Sedimentary Structures and Depositional Environments of Diatomaceous Strata of Sisquoc Formation, Santa Maria Area, California: ABSTRACT

A gravity model of the basement structure in the Santa Maria Basin, California

The Santa Maria Basin in southern California is a lowland bounded on the south by the Santa Ynez River fault and on the northeast by the Little Pine‐Foxen Canyon‐Santa Maria River faults. It contains Neogene sedimentary rocks which rest unconformably on a basement of Cretaceous and older clastic rocks. Analysis of over 4 000 gravity stations obtained from the Defense Mapping Agency suggests that the Bouguer anomaly contains a short‐wavelength component arising from a variable‐density contrast between the basin’s Neogene units and the Cretaceous basement. A three‐dimensional inversion of the short‐wavelength component (constrained by wells drilled to basement) yields a structure model of the basement and the average density of the overlying sediments, assuming that the basement does not contain large‐scale density variations. The density anomalies modeled in the Neogene sediments, showing higher densities in the basin troughs, can be related to diagenetic changes in the silica facies of the Monterey and Sisquoc formations. The basement structure model shows the basin as composed of parallel ridges and troughs, trending west‐northwest and bounded by steep slopes interpreted as fault scarps. The basin is bounded on the west by a north‐south trending slope which may also represent a fault scarp.

Diatom Stratigraphy of the Lower Pliocene Part of the Sisquoc Formation, Harris Grade Section, California

Lower Pliocene diatoms were studied from the Sisquoc Formation and lowermost Foxen Mudstone, exposed along Harris Grade north of Lompoc, California, to refine the diatom biostratigraphy of post-Monterey Formation sediments in California. Sixty-seven diatom taxa were identified in the 25 samples examined from the 790-m thick (2950-ft) section. The diatoms are assignable to the uppermost Nitzschia reinholdii Zone and Thalassiosira oestrupii Zone of Barron (1981), and five tentative subzones for local correlation are proposed. Regional correlations and taxon occurrence are discussed, and the base of the Nitzschia reinholdii Zone is redefined as at the last occurrence of Thalassionema schraderi.

Hemipelagic deposits in a Miocene basin, California: toward a model of lithologic variation and sequence

Summary Deep marine basins began to form along the continental margin of central California in late Oligocene time as a result of the initiation of a translational plate boundary. Deposition of transgressive shallow-marine sand (Vaqueros Formation) was followed by deposition of a thick (> 500 m) massive clay unit (Rincon Shale). Rapid subsidence (> 2000 m) coincident with rising sea-level created sediment-starved basins by late early Miocene time ( c. 18 Ma). Typical of many of these borderland basins was the Santa Barbara basin, located at least 50 km from the Miocene strandline. In this basin, between late early and latest Miocene time (18–5.5 Ma), was deposited a heterogeneous sequence of highly biogenous sediment (Monterey Formation) consisting of mixed diatomaceous, foraminiferal-coccolithic, and terrigenous debris generally rich in organic matter (mean 8%). Overlying latest Miocene and Pliocene sediments (Sisquoc Formation) contain increasingly abundant clay debris. The Miocene biogenous sequence reflects hemipelagic sedimentation from highly productive coastal waters during an extended period of generally low terrigenous influx. Accumulation of terrigenous debris decreased markedly in the early Miocene (from 11 to 1 g/cm 2 ·10 3 yr) and was low throughout the middle Miocene, increasing somewhat at about 8 Ma and markedly after 5.5 Ma; this pattern is partly related to global sea-level changes but also resulted from high rates of subsidence and tectonic events. Biogenous silica accumulation peaked (mean 2.5 g/cm 2 ·10 3 yr) in the late early Miocene (18–15 Ma) and again in the late Miocene and early Pliocene (8–3.5 Ma) and was 5 to 25 times slower in the interval 15–11 Ma; these variations are thought to reflect primarily variations in the intensity of upwelling and associated production of diatoms. Accumulation rates of biogenous calcite gradually declined from a late early Miocene peak through the early late Miocene and virtually ceased in latest Miocene (8–5.5 Ma), probably due to dissolution. Massive stratification indicates that bottom waters were oxygenated until latest early Miocene time ( c. 16 Ma), when expansion of the oxygen-minimum zone or tectonic formation of a sill within the oxygen-minimum zone produced marked lowering of oxygen levels resulting in the dominance of laminated stratification between 16 and 5.5 Ma. Abundance of organic matter is closely associated with the abundance of clay and of calcite but is inversely correlated with silica content. Organic matter is also about twice as abundant in massive or discontinuously laminated beds (indicating good to moderate oxygenation) than in associated beds with varve-like lamination (indicating minimal oxygenation). These relations indicate that grain-size outweighed the influence of low-oxygen bottom waters in preserving organic matter.

Specialized Pumping Techniques Applied to a Very Low-Gravity, Sand-Laden Crude - Cat Canyon Field, California

Summary With the application of specially designed pumping and treating equipment, we are producing crude oil as low as 4°API (1.044 g/cm3) containing up to 70 vol% of sand. An important part of this system is the bottom hole pumping equipment that has allowed primary production rates in excess of 150 B/D (24 m3/d) oil from wells that were restricted to less than 10 B/D (1.5 m3/d) oil when produced with conventional rod pumps and sand control completion methods. These pumps are available commercially. Techniques developed may be extended to other areas where production is limited by problems inherent with highly viscous crude oils and excessive sand entry. Introduction Problems with pumping extremely viscous crude oils are well known to all operators who produce low-gravity crude oil. Rod floating, overloaded pumping units, and pumps plugged with heavy crude oil are common occurrences. Likewise, such problems as stuck pumps, eroded tubing and sucker rod couplings, and stripping jobs are common wherever sandy production occurs. We experienced all these problems in producing the Brooks reservoir, located in the Cat Canyon field, Santa Barbara County, CA, where oil gravity averages 6°API (1.029 g/cm3), and the production contains up to 70% sand. Over a 5-year period these problems were ameliorated with the development and successful application of special downhole pumping equipment. This paper up-dates a previous work on this topic.1 The Brooks reservoir sand is an unconsolidated basal member of the Pliocene Sisquoc formation. The sand is very fine-grained and well-sorted, with a medium sand-grain size of 0.0065 in (0.165 mm) and a Trask sorting coefficient of 1.2. Reservoir depth ranges from 2,500 to 3,500 ft (760 to 1070 m); average net sand thickness is 150 ft (46 m). A type log is shown in Fig. 1. There is no significant primary water production, and the zone produces at less than 1 to 2 % water cut, except where wateror steam injection has occurred. The oil gravity of Brooks zone is among the lowest produced in the world, and ranges from 11 °API (0.993 g/cm3) in the upstructure area to about 0°API (1.076 g/cm3) downstructure at the reservoir limits. Reservoir oil currently averages about 6°API (1.029 g/cm3) and has 15,000-cp (15-Pa·s) viscosity at reservoir conditions. Reservoir properties of the Brooks sand and fluids are summarized in Table 1. Operating Experience With Conventional Equipment Production from the Brooks zone began in 1909 in the area where highest oil gravity occurs. These early wells were completed with 250-mesh [0.25-in. (6.35-mm)] slots or 1/2-in. (12.7-mm) holes through the zone, which allowed almost unrestrained sand production from the unconsolidated formation. Initially, these wells flowed up to400 B/D (64 m3/d) of 10°API (1.0 g/cm3) oil containing about 15 % sand. Cumulative recovery from this old part of the field is now more than 10% of the OOIP. Individual wells have produced over 400,000 bbl (64 000 m3) of oil and 40,000 bbl (6400 m3) of sand over a 40-year life. To pump the highly viscous oil with conventional pumping equipment it was necessary to inject a distillate into the wellbore, which blended the native oil up to about 12°API (0.968 g/cm3).

Correlations Between the Onshore and Offshore Santa Maria Basins -- A Dilemma: ABSTRACT

In an unexplored basin, extrapolation from known hydrocarbon-producing trends is ideal. However, along transform margins such extrapolations are difficult, owing to lateral displacement between individual blocks on both regional and local scales. An example of this is the relationship between the onshore and offshore Santa Maria basins, which are separated by the Hosgri fault. Stratigraphic packages have been used widely to approximate amounts of displacement. Lower Miocene volcanics in the offshore Santa Maria P-060-Oceano well may correlate with onshore outcrops, located across the Hosgri fault, 30 mi (48 km) to the east and 45 mi (72 km) to the south, near Point Arguello. Additionally, lower Miocene volcanics also are present in two exploratory wells across the Hosgri fault, 10 mi (16 km) to the east and 25 mi (40 km) to the north, near Point Buchon. These are the Honolulu-Tidewater U. S. L. Heller Lease 1, with 4,722 ft (1,440 m) thickness of volcanics, and the Tidewater Motadoro 1, with 3,873 ft (1,180 m) of lower Miocene volcanics. These wells provide two volcanic sections onshore to tie with the offshore volcanics. Originally, the lower Miocene volcanics now situated in the northern and southern extremities of the Santa Maria area, may have been joined near the midpoint of their present positions. As the onshore basin pulled apart, the volcanics were divided and transported in opposite directions. Synchronous pull-apart movements occurring in the offshore kept pace with the adjacent onshore. Alternatively, significant intrusive pathways may have opened in the later stage of basin development, allowing igneous material to migrate vertically. These pathways have been termed leaky transforms in the literature. Neither of these models necessitates significant lateral displacement once the onshore and offshore basins formed. Onshore the middle Miocene Monterey Formation and upper Miocene to Pliocene Sisquoc Formation correlate well with equivalent chronostratigraphic units in the offshore P-060-Oceano well, implying that relatively minor lateral displacement has occurred since the middle Miocene. If the offshore basin history is similar to that of the onshore its petroleum potential may approximate that of the onshore, which has been projected to produce 900 million bbl of oil. End_of_Article - Last_Page 1688------------