Fine-scale Heterogeneities Research Articles

Construction of Reservoir Flow Models Using Deterministic and Stochastic Methods and Driven by Seismic Acoustic Impedance -- a Gulf of Mexico Example: ABSTRACT

Rapid and accurate subsurface descriptions and flow predictions contribute to profitable field development, particularly for deep water fields of modest size. This paper shows how 3-D seismic acoustic impedance played an essential roll in that process. This Tertiary field example exhibits a classic Gulf of Mexico {open_quotes}bright{close_quotes} seismic response. The slope channel deposits yielded reflections from stacked pay zones that were often interfering vertically and laterally variable. Seismic acoustic impedance inversion for volume estimation, well placement and flow model construction. Flow model construction was facilitated using Stratamodel, where reservoir boundaries, porosity and permeability were estimated from seismic acoustic impedance. This provided a {open_quotes}deterministic{close_quotes} flow model with which well choices and development economics were explored. Alternate flow models were developed in which the effect of fine scale (sub-seismic) heterogeneities were investigated. A 3-D {open_quotes}stochastic{close_quotes} model was developed that honored geostatistical parameters as well as seismic acoustic impedance. This gave insight to permeability distributions and confirmed that connectivity between scattered sand bodies would not significantly degrade the field performance predicted by deterministic models.

The Use of Categorical Indicator Geostatistics for Modeling Facies in Sand-Rich Turbidite Systems: An example from the Deep-Water Gulf of Mexico: ABSTRACT

The Green Canyon 161/205 prospect, located 150 miles offshore Louisiana in the central Gulf of Mexico at 2500-3000 ft water depths, has between 350-425 million barrels of original oil in place that is stratigraphically and structurally trapped within lower Pleistocene and upper Pliocene turbidites. Overall sand content within the major pay zones is greater than 80%. The vertical lithofacies variations, seismic facies maps, and architecture of the primary reservoirs in the prospect are characterized using data available from conventional well logs, whole core, pressure transient tests, and a 3-D seismic survey. Due to the high costs associated with developing such a prospect, an extensive 3-D reservoir simulation evaluation program was employed. In this study we outline the role of geostatistical methods in the creation of these 3-D simulation models. A two-stage approach was used to capture the effects of geological heterogeneity on fluid flow. In the first stage, large-scale seismic facies variations, which could be identified through correlations between log interval averages and seismic amplitudes, were first identified and mapped for each reservoir zone. The seismic facies maps were first sampled as point values at pseudo-well locations. Information from these selected points assisted in creating {open_quote}facies indicator variograms{close_quotes} for eachmore » facies identified in the seismic maps. Next, the pseudo-wells, along with the sparse actual wells, were treated as {open_quote}hard{close_quotes} data points in geostatistical simulations of equi-probable facies scenarios. The second stage of model construction involved translating fine-scale heterogeneities observed in core and logs into synthetically-generated traces of turbidite sedimentation. These synthetic traces were then combined with lateral correlation length scales obtained from outcrop studies to condition geostatistical cross-sections of porosity and permeability.« less

Finite difference modeling of geoacoustic interaction at anelastic seafloors

A major problem in understanding seismic wave propagation in the seafloor is to distinguish between the loss of energy due to intrinsic attenuation and the loss of energy due to scattering from fine scale heterogeneities and bottom roughness. Energy lost to intrinsic attenuation (heat) disappears entirely from the system. Energy lost to scattering is conserved in the system and can appear in observations as incoherent noise (reverberation, time spread, angle spread) and/or mode converted waves. It has been shown by a number of investigators that the seafloor scattering problem can be addressed by finite difference solutions to the elastic wave equation in the time domain. However previous studies have not considered the role of intrinsic attenuation in the scattering process. In this paper, a formulation is presented which includes the effects of intrinsic attenuation in a two-dimensional finite difference formulation of the elastodynamic equations. The code is stable and yields valid attenuation results.

Isotope and chemical microsampling: Constraints on the history of an S-type rhyolite, San Vincenzo, Tuscany, Italy

Fine scale heterogeneities in strontium isotope ratios, 40Ar- 39Ar ages, and chemical composition have been determined for individual mineral grains and enclaves in the rhyolites of San Vincenzo, Italy. The rhyolites are peraluminous with phenocrysts of alkali feldspar, plagioclase, biotite, and cordierite and have previously been divided into two groups on the basis of whole rock major element chemistry. Group B lavas are distinguished from Group A in having higher MgO and CaO contents as well as containing two pyroxenes and chilled latite enclaves. The Group B lavas show textural evidence for disequilibrium, such as resorption and sieve zones in feldspars. Laser fusion 40Ar- 39Ar dating of individual alkali feldspar crystals suggests that the eruption of these rhyolites took place 4.38 ± 0.04 Ma, although small amounts of excess argon are present in subhedral and cloudy crystals. The groundmass in the Group A sample has a higher initial 87Sr 86Sr (0.725) than that of the Group B (0.713). The Sr isotope compositions of chemically and texturally characterized individual grains reveal marked disequilibrium between minerals and the host glass as well as within-grain zonation. The feldspars and coarse-grained biotites have initial 87Sr 86Sr that are intermediate between those of the Group A and B glasses, with alkali feldspar generally having higher initial 87Sr 86Sr ratios than plagioclase. Chilled latite enclaves, which are present in only the Group B lavas, have relatively low initial 87Sr 86Sr ratios in the range 0.7082–0.7088. The mineral chemistry and isotope data indicate that restite does not form a significant portion of the crystalline assemblage and, therefore, cannot be the source of the strontium isotope disequilibrium. Xenocrysts mixed into the rhyolite from a mafic magma were identified as clinopyroxene megacrysts, clinopyroxene-orthopyroxene clots, orthopyroxene-plagioclase clots, and plagioclase with extensive sieve textures. These grains all have relatively low initial 87Sr 86Sr ratios. Other grains that may be xenocrysts are cordierites with cores containing inclusions of biotite, spinel, and sillimanite, and alkali feldspars with excess 40Ar. The strontium isotope disequilibrium in phenocrysts of plagioclase, alkali feldspar, and biotite is attributed to a changing melt chemistry that was too rapid for strontium diffusive equilibration. A model for the San Vincenzo magma chamber is proposed in which the Sr isotope evolution of the Group B rhyolites is due to magma mixing of a crustal melt with latite magma containing unradiogenic strontium. The variable initial strontium isotope compositions of Group A phenocrysts may be due to assimilation or crystallization along the interface between mingled Group A and B magmas.

Evaluation and application of the ion microprobe in the strontium isotope geochemistry of carbonates

A modified AEI-IM20 ion microprobe has been used to measure 87Sr/ 86Sr ratios in carbonates. A suite of carbonates with varying major elements (Ca, Mg, Fe, Mn) was studied at low and high ( M/ΔM ⋍ 3000) mass resolution to determine the types and intensities of molecular species isobaric with Sr peaks; Sr data collected at low mass resolution must be corrected for Ca 2 and CaMgO species. Rb/Sr ratios are extremely low, and correction for 87Rb is not required (< 0.1‰ of 87Sr). Usable Sr isotopic data may be obtained from calcite given Sr≥ 400ppm, and for Sr > 5000 ppm a precision of ∼ ± 1‰ (± 0.0007) in 87Sr/ 86Sr (2σ mean) can be achieved under optimum conditions. The corrections for Ca 2 and CaMgO are smaller than the within-run precision in calcite, but in dolomite the correction for CaMgO is + 1.5%. Mass fractionation corrections to 87Sr/ 86Sr (based on 86Sr/ 88Sr= 0.1194) are typically +8 to +10‰. Good agreement between ion probe and solid source mass spectrometer results was found for calcites of known Sr isotopic composition: ST4a (∼ 400ppm Sr), average ion probe 87Sr/ 86Sr= 0.7267 ± 0.0015, solid source 87Sr/ 86Sr= 0.72680 ± 0.00008 [14]; JCG36 (∼ 6000ppm Sr), average ion probe 87Sr/ 86Sr= 0.7056 ± 0.0009, solid source 87Sr/ 86Sr= 0.70588 ± 0.00009 [16]; JCG44 (∼ 6000ppm Sr), average ion probe 87Sr/ 86Sr= 0.7057 ± 0.0006, solid source 87Sr/ 86Sr= 0.70540 ± 0.00008 [16]. The ability of the ion microprobe to measure 87Sr/ 86Sr for 10-μm spots in calcite was used: (1) to measure variation in 87Sr/ 86Sr of ∼ 0.01 on a centimetre scale in a hydrothermally altered basalt from the Isle of Skye, northwestern Scotland; and (2) to determine the Sr isotopic composition of tiny (< 35 μm) calcite grains in a veined mantle lherzolite from Bultfontein, South Africa. Because of calcite's ubiquitous occurrence in many parageneses this technique may offer many opportunities for the measurement of fine scale heterogeneities in 87Sr/ 86Sr.