Low Fluid Pressure Research Articles

Cementation-Dissolution Timing Relative to Hydrocarbon Emplacement, San Joaquin Basin, California: ABSTRACT

Calcite cements occur as thin bands (up to 1 m thick by 200 m long) in a Stevens turbiditic sandstone (upper Miocene) reservoir at North Coles Levee. These cements formed from evolved marine power water and show up to 10 mole % of mainly Fe + Mg. Stable isotopic analysis and estimated isotopic compositions of pore fluid indicate the cements formed from 40/sup 0/ to 80/sup 0/C and that delta/sup 13/C ratios range from +1.2 to -12.66. A burial history curve indicates the cements formed from 7.5 to 3.5 Ma. Dissolution of calcite, widespread dissolution of plagioclase (up to one-third total porosity), precipitation of kaolinite, and migration of hydrocarbon (500 million bbl) occurred after this time. Hence considerable mass transfer is implied over a relatively short period of time, and oil migration closely followed the dissolution event. Increased concentrations of acids during kerogen maturation as observed by Carothers and Kharaka in 1978 for organic acid species at 80/sup 0/ to 100/sup 0/C in present-day pore waters of the basin may have caused the dissolution process. Very recent, possible post-oil migration processes in the field include albitization of plagioclase and dolomite rhombs growing in crushed biotites. These stress-related processes may bemore » the indirect result of low fluid pressures (1000 psi at 8500 ft) from release of the reservoir gas cap in 1965. At present, calcite with relatively high magnesium contents (to 8 mole %) precipitate as scales in the tubing string, presumably a result of CO/sub 2/ degassing where pressure drop occurs in the flow line.« less

Dynamics of an End-On Collision, Interpreted from SeaMARC II Swath Mapping and Migrated Seismic Reflection Data on North Margin of Panama: ABSTRACT

The authors have imaged nearly the entire North Panama thrust belt (NPTB) to determine the dynamics of the Panama arc colliding end-on with South America. Observations support a simple flexural beam model in which eastward collision of the arc against the continent is transformed into northward relative motion of the NPTB. Fold axes and thrust faults within the NPTB show remarkable uniformity in strike (075/sup 0/), changing only on the east margin across a major fault and on the west margin across a sharp flexure. The belt increases in width (and presumably convergence) eastward, as predicted by the beam model. Displacement along the Uraba fault zone separated a thin sliver of the NPTB, and two earthquake focal mechanisms on the fault show right-oblique slip. Vergence of thrust sheets within the accretionary wedge is dominantly landward in much of the central part of the thrust belt, indicating low basal shear stresses and high fluid pressures along the decollement. Mud volcanoes within the accretionary wedge attest to elevated fluid pressures. Because of an exceptionally well-developed and widespread gas hydrate layer, they suggest that pore fluids are a mixture of water and methane. However, near the central part of the thrust belt, amore » concentrated zone of mud volcanoes has breached the crests of seaward-verging anticlines. They propose that the zone of anomalous seaward-verging thrust sheets results from widespread release of pore fluids from the decollement, leading to increased basal friction.« less

Buckling prevention by lateral fluid pressure in deep-drawing

An instability analysis of flange buckling against lateral fluid pressure in deep drawing is considered. It is intended to explain the experimental fact that relatively low fluid pressure when applied to the flange area can suppress buckling. The analysis is based on the approximate ‘energy method’ with the inclusion of the work against the fluid. The attention is focused on the initiation of the deep drawing process, where buckling (of non-hardening material) is most susceptible. A special apparatus which enables the replacement of a rigid blank-holder by a lateral fluid-pressure was used for testing. A general solution to the critical pressure, above which the deep drawing can be terminated without buckling, is provided. The prediction of the critical pressure and the number of the associated buckling ‘waves’ (wrinkles) agree very well with the experiments. The pertinent geometrical and material variables (as blank thickness, drawing ratio, Young modulus, yield strength, etc.) are grouped in nondimensional form and plotted for various parameters to provide an engineering-type solution for potential users.

Alkali-free tourmaline in the system MgO-Al 2O 3-B 2O 3-SiO 2-H 2O

An end member of the tourmaline series with a structural formula □(Mg 2Al)Al 6(BO 3) 3[Si 6O 18](OH) 4 has been synthesized in the system MgO-Al 2O 3-B 2O 3-SiO 2-H 2O where it represents the only phase with a tourmaline structure. Our experiments provide no evidence for the substitutions Al → Mg + H, Mg → 2H, B + H → Si, and AlAl → MgSi and we were not able to synthesize a phase “Mg-aluminobuergerite” characterized by Mg in the (3a)-site and a strong (OH)-deficiency reported by Rosenberg and Foit (1975). The alkali-free tourmaline has a vacant (3a)-site and is related to dravite by the □ + Al for Na + Mg substitution. It is stable from at least 300°C to about 800°C at low fluid pressures and 100% excess B 2O 3, and can be synthesized up to a pressure of 20 kbars. At higher temperatures the tourmaline decomposes into grandidierite or a boron-bearing phase possibly related to mullite (“B-mullite”), quartz, and unidentified solid phases, or the tourmaline melts incongruently into corundum + liquid, depending on pressure. In the absence of excess B 2O 3 tourmaline stability is lowered by about 60°C. Tourmaline may coexist with the other MgO-Al 2O 3-B 2O 3-SiO 2-H 2O phases forsterite, enstatite, chlorite, talc, quartz, grandidierite, corundum, spinel, “B-mullite,” cordierite, and sinhalite depending on the prevailing PTX-conditions. The (3a)-vacant tourmaline has the space group R3m with a =15.90 A ̊ , c = 7.115 A ̊ , and V = 1557.0 A ̊ 3 . However, these values vary at room temperature with the pressure-temperature conditions of synthesis by ±0.015 A ̊ in a , ±0.010 A ̊ in c , and ±4.0 A ̊ 3 in V , probably as a result of MgAl order/disorder relations in the octahedral positions. Despite these variations intensity calculations support the assumed structural formula. Refractive indices are n o = 1.631(2), n E = 1.610(2), Δn = 0.021. The infrared spectrum is intermediate between those of dravite and elbaite. The common alkali and calcium deficiencies of natural tourmalines may at least partly be explained by miscibilities towards (3a)-vacant end members. The apparent absence of (3a)-vacant tourmaline in nature is probably due to the lack of fluids that carry boron but no Na or Ca.

Erosion and the mechanics of shallow foreland thrusts

A simple model for the effects of erosion on the geometry and mechanics of shallow foreland thrusts allows prediction of maximum stable length of the thrust toe, location of initial failure within the toe, if total thrust displacement exceeds the maximum stable length, and estimates of average pore fluid pressures along the fault during thrust advance. For ‘typical’ rates of erosion and thrust advance, the maximum stable length of the toe may be very large (over 50 km) for relatively low values of fluid overpressure. Application of the theoretical results to the Keystone-Muddy Mountain thrust of southern Nevada predicts: maximum stable length, location and degree of imbrication in agreement with observation; relatively low pore fluid pressures along the fault (λ between 0.5 and 0.6), and a rate of thrust advance between 4 and 8 km Ma −1.

Petrology of a portion of the Eastern Peninsular Ranges mylonite zone, Southern California

Peraluminous and metaluminous plutonic rocks of the Peninsular Ranges batholith near Borrego Springs in southern California were mylonitized in the large shear zone known as the eastern Peninsular Ranges mylonite zone (EPRMZ). Accompanying mylonitization in this portion of the EPRMZ was metamorphism at intermediate-low-pressure amphibolite-facies conditions. Deformation in the zone overlapped in time with Cretaceous intrusion of the batholith. In the San Ysidro Mountain — Pinyon Ridge area, four north-south trending zones of differing intensity of deformation have been defined; from east to west the degree and style of deformation gradually change from undeformed or weakly deformed rocks to strongly mylonitized rocks. Electron microprobe analysis shows that recrystallized hornblende, biotite, and plagioclase are variable in composition, probably reflecting a range of metamorphic conditions accompanying deformation. Comparison of mineral compositions with those from mafic schists of Vermont suggests conditions ranged from andalusite-staurolite through sillimanite-muscovite grades as defined for pelitic rocks. Stability of muscovite+quartz in mylonite assemblages and lack of remelting of granitic rocks indicate that temperature did not exceed about 650° C during mylonitization and lithostatic pressure did not exceed about 5 kbar. Over time, any given rock volume experienced a range of temperature, lithostatic pressure, and perhaps fluid pressure and differential stress. Mineral reactions in the zone involved hydration, requiring introduction of water. The possibility of large-scale migration of K and Fe is suggested by whole-rock chemical data. Brittle and ductile deformation features are closely associated in one part of the EPRMZ. The combined evidence suggests the presence of a pore fluid with fluid pressure close to lithostatic pressure. Short periods of low fluid pressure and possible high differential stress cannot be ruled out.

Hydrodynamics of Denver Basin, An Explanation of Subnormal Fluid Pressures: ABSTRACT

Anomalously low fluid pressures are found in the Lower Cretaceous, Mesozoic, and Paleozoic rocks of the Denver basin. Drill-stem test data and published hydrogeologic information are used to construct a potentiometric map for the Lower Cretaceous sandstones in the area. Normally, one expects the potential surface to be at or near the land surface (0.43 psi/ft). However, the potential surface for the Lower Cretaceous sandstones and underlying Paleozoic rocks is up to 2,500 ft (762 m) beneath the land surface (0.35 psi/ft) in parts of the Denver basin in Colorado and the Nebraska panhandle. The low pressures seem especially anomalous considering the elevation of the outcrops along the Rocky Mountain Front and the Black Hills. The hydrostratigraphy is defined based on the known regional geology. Structure, isopach, and lithofacies maps are used to estimate the hydraulic characteristics of the rocks in the basin. A numerical model is constructed, based on the hydrostratigraphy, which simulates the regional flow system. Both transient and steady-state flow regimes are simulated. The interaction of the Lower Cretaceous sandstones with overlying and underlying hydrostratigraphic units is investigated. The significance of recharge in the outcrop areas is evaluated. The model is used to define the conditions under which subnormal fluid pressures may occur. The subnormal fluid pressures are reasonably explained as a consequence of regional ground-water flow. End_of_Article - Last_Page 422------------

Pressure, Temperature, Salinity, Lithology, and Structure in Hydrocarbon Accumulation in Constance Bayou, Deep Lake, and Southeast Little Pecan Lake Fields, Cameron Parish, Louisiana: ABSTRACT

ABSTRACT Pressure, temperature, salinity, lithology, and structural studies indicate that hydrocarbons in Deep Lake, Constance Bayou, and Little Pecan Lake fields, were generated in the shale beds of the hard geopressured zone and migrated upwards along major growth faults. The hydrocarbons were originally dissolved in hot fresh pore water and came out of solution in the overlying low temperature and pressure zones, accumulating in the sand beds of the first structural traps encountered. By examining regional cross sections and anomaly maps, fluid escape routes taken by the hot pore water containing dissolved hydrocarbons can be identified. Areas below which a vertical flush of hot fresh pore water from the hard geopressured zone has occurred have three identify characteristics: low fluid pressures, high formation water salinity values, and presence of residual high pressure areas. These areas are considered to be highly prospective places to search for hydrocarbon accumulations. There are five areas in the study area below which a vertical flush has occurred from the hard geopressured zone and each contains commercial accumulations of hydrocarbons. This report concludes that pressure, temperature, and salinity studies, when coupled with lighology and structure, add a new dimension to hydrocarbon exploration and should definitely be used in the search for new reserves of oil and gas.

Pressured Shale and Related Sediment Deformation: Mechanism for Development of Regional Contemporaneous Faults

Regional contemporaneous faults of the Texas coastal area are formed on the seaward flanks of deeply buried linear shale masses characterized by low bulk density and high fluid pressure. From seismic data, these masses, commonly tens of miles in length, have been observed to range in size up to 25 mi in width and 10,000 ft vertically. These features, aligned subparallel with the coast, represent residual masses of undercompacted sediment between sandstone-shale depoaxes in which greater compaction has occurred. Most regional contemporaneous fault systems in the Texas coastal area consist of comparatively simple down-to-basin faults that formed during times of shoreline regression, when periods of fault development were relatively short. In cross-sectional view, faults in hese systems flatten and converge at depth to planes related to fluid pressure and form the seaward flanks of underlying shale masses. Data indicate that faults formed during regressive phases of deposition were developed primarily as the result of differential compaction of adjacent sedimentary masses. These faults die out at depth near the depoaxes of the sandstone-shale sections. Where subsidence exceeded the rate of deposition, gravitational faults developed where basinward sea-floor inclination was established in the area of deposition. Some of these faults became bedding-plane type when the inclination of basinward-dipping beds equaled the critical slope angle for gravitational slide. Fault patterns developed in this manner are comparatively complex and consist of one or more gravitational faults with numerous antithetic faults and related rotational blocks. Postdepositional faults are common on the landward flanks of deeply buried linear shale masses. Many of these faults dip seaward and intersect the underlying low-density shale at relatively steep angles. Conclusions derived from these observations support the concept of regional contemporaneous fault development through sedimentary processes where thick masses of shale are present and where deep-seated tectonic effects are minimal.

Pressured Shale and Related Sediment Deformation--Mechanism for Development of Regional Contemporaneous Faults: ABSTRACT

Regional contemporaneous faults of the Texas coastal area are formed on the seaward flanks of deeply buried linear shale masses characterized by low bulk density and high fluid pressure. From seismic data, these masses, commonly tens of miles in length, have been observed to range in size up to 25 mi in width and 10,000 ft vertically. These features, aligned subparallel with the coast, represent residual masses of undercompacted sediment between sandstone-shale depoaxes in which greater compaction has occurred. Most regional contemporaneous fault systems in the Texas coastal area consist of comparatively simple down-to-basin faults that formed during times of shoreline regression, when periods of fault development were relatively short. In cross-sectional view, faults in hese systems flatten and converge at depth to planes related to fluid pressure and form the seaward flanks of underlying shale masses. Data indicate that faults formed during regressive phases of deposition were developed primarily as the result of differential compaction of adjacent sedimentary masses. These faults die out at depth near the depoaxes of the sandstone-shale sections. Where subsidence exceeded the rate of deposition, gravitational faults developed where basinward sea-floor inclination was established in the area of deposition. Some of these faults became bedding-plane type when the inclination of basinward-dipping beds equaled the critical slope angle for gravitational slide. Fault patterns developed in this manner are comparatively complex and consist of one or more gravitational faults with numerous antithetic faults and related rotational blocks. Postdepositional faults are common on the landward flanks of deeply buried linear shale masses. Many of these faults dip seaward and intersect the underlying low-density shale at relatively steep angles. Conclusions derived from these observations support the concept of regional contemporaneous fault development through sedimentary processes where thick masses of shale are present and where deep-seated tectonic effects are minimal.

Pressure-Depth Relations in Appalachian Region

The pre-Pennsylvanian reservoirs of the Appalachian region are predominantly lenticular. Fluid pressures in many gas fields in these reservoirs are subnormal, but some are normal and a few are above normal. Each reservoir, across large areas, has a characteristic departure from normal of the mean values of the fluid pressures. Abnormally low fluid pressures tend to occur in lenticular reservoirs closely associated with shales in areas which have undergone erosion. A possible explanation is that erosion causes a reduction in the fluid pressure in the pore space of shales, and that this reduction is transmitted to the closely associated reservoir rocks. The pressure reduction in shales may be due to the increase in pore volume and absorption of water in clay minerals as the overburden pressure decreases, and to the absorption of water during mineral transformations that occur because of the decrease in temperature.

Stability relations of K2Mg5Si12O30, and end member of the merrihueite-roedderite group of meteoritic minerals

The phase K2Mg5Si12O30 was synthesized both hydrothermally and dry under a variety of pressures and temperatures, and its stability relations were determined. Under hydrothermal conditions it exhibits a lower stability limit lying at 595°C, 1 kb, and 650°C, 2 kb, due to its breakdown into the hydrous assemblage quartz+KMg2.5Si4O10(OH)2 (a mica phase). Its upper temperature stability under hydrothermal conditions is given by its incongruent melting to MgSiO3+liquid. Near 820° C at a fluid pressure of approximately 6.5 kb the two univariant curves for these breakdown reactions intersect thus limiting the stability field to lower fluid pressures. — Under anhydrous conditions K2Mg5Si12O30 becomes unstable at pressures between approximately 7 and 32.5 kb due to its incongruent melting to the assemblage MgSiO3+quartz (or coesite)+liquid; this melting curve has a pronounced negative slope. No subsolidus breakdown assemblage was encountered at 32.5 kb down to temperatures as low as 750°C. This behavior is probably due to the instability of other ternary compounds in the system K2O-MgO-SiO2 at high pressures and thus to the existence of very low-temperature eutectics involving only binary and unary solid phases plus liquid.

197. Clinical Observation of Spontaneous Low Spinal Fluid Pressure Syndrome

197. Clinical Observation of Spontaneous Low Spinal Fluid Pressure Syndrome

Low spinal fluid pressure syndromes.

Low spinal fluid pressure syndromes.

Laboratory Study of Effect of Overburden, Formation and Mud Column Pressures on Drilling Rate of Permeable Formations

Published in Petroleum Transactions, AIME, Volume 217, 1959, pages 9–17. Abstract One phase has been completed of a laboratory investigation of formations with relatively high permeability under conditions of overburden, formation and mud column pressures. The following statements are based on these tests within the limitations described. Drilling rate decreased when mud column pressure was greater than formation pressure. The decrease was primarily due to a layer of cuttings and mud particles held to the hole bottom by the difference in pressure. Much of the force of the bit was wasted in this layer and was unavailable for penetrating virgin formation. Adequate jet velocities helped clean the filter cake and chips away and resulted in increased drilling rate the higher the jet velocity the faster the drilling rate. Drilling rate increased slightly when formation pressure was greater than mud column pressure. The increase resulted from cleaning the hole bottom as formation fluid flowed into the borehole. Overburden pressure had practically no effect on drilling rate. Introduction Some formations are difficult to drill in a wellbore hole but are easily drilled under atmospheric conditions in the laboratory. The reasons for this apparent difference in drill ability are not fully understood. An increase in formation compressive strength causes a reduction in drilling rate. It has been shown by Griggs, Handin, and others that formation strength increases under conditions of high hydrostatic stress. Based on these results, a decrease in drilling rate would be expected with increased drilling fluid pressure in formations of low permeability having low formation fluid pressure. Such a decrease has been shown by Murray, et al and Eckel. This effect holds even when using nitrogen under high pressure as the drilling fluid (unreported tests from the Hughes laboratory). Poor bottom-hole cleaning may account for decreased drilling rates. Even though it may not be easily explained, evidence of poor cleaning can easily be seen.

Clinical significance of low cerebral spinal fluid pressure

Clinical significance of low cerebral spinal fluid pressure

Coelomic pressures in sipunculus nudus.

The relationship between coelomic pressures and body movements has been investigated in Phascolosoma gouldi and, mainly, Sipunculus nudus.During "defense movements, pressure in Phascolosoma is commonly 50 cm. of water, in Sipunculus 70 to 80 cm. During "defense immobility," the highest pressure recorded, in Phascolosoma, was 108 cm. There does not seem to be a relation between body size and maximum pressure.Six phases of burrowing movements have been distinguished in Sipunculus and related to coelomic pressure. An important pressure rise coincides with a bending movement of the central part the body. This "hump" and the high pressures associated with it have functions which are described. Very low coelomic fluid pressures suffice to cause evension of the proboscis. Pressure is a necessary and sufficient condition for proboscis eversion in free water, while in sand the proboscis muscles seem to play an essential role during proboscis eversion in relation to the thixotropic property of stirred sand.During most experiments, minimum pressures were very constant, while maximum pressures varied a great deal. During burrowing movements in free water, the highest pressure recorded in Phascolosoma was 25.0 cm., in Sipunculus 39.6 cm. of body fluid. During burrowing in sand, pressure of 96 cm. water was reached by Sipunculus.The frequency of pressure cycles seems to be independent of the absolute pressure.The coordination between the cycles of pressure and proboscis movements is variable. Therefore the retractor muscles and the muscles of the body wall are probably dependent on nervous centers which are not closely associated.Pressure variations during burrowing have an influence on circulation of coelomic fluid.