Muddy Reservoirs Research Articles

Experimental Measurements and Empirical Relationships on Permeability Evolutions of Water/Hydrate-Containing Muddy Reservoirs

Experimental Measurements and Empirical Relationships on Permeability Evolutions of Water/Hydrate-Containing Muddy Reservoirs

Numerical simulation on gas production from inclined layered methane hydrate reservoirs in the Nankai Trough: A case study

Natural gas hydrate is generally disseminated in tilted sediments. The inclination of hydrate reservoirs significantly affects the fluid migration behaviour and production potential. However, most of the current simulations ideally assume that the hydrate reservoir is horizontally distributed; resulting in the effect of the formation dip on the well deployment and corresponding gas production remains unclear. Here, we implement a real inclined alternating sand–clay hydrate reservoir model based on geological data from the Nankai Trough, Japan, to simulate the production performance of a single horizontal well and dual horizontal wells. Results show that the gas and water productions are enhanced when a single horizontal production well is deployed in the structural low of the reservoir. The gas and water production performance in upper sandy reservoir and the water extraction in the middle muddy reservoir are sensitive to the layout of the production wells, which can cause the daily difference is over 6%. The gas-to-water ratio is independent from the structural deployment positions of the horizontal wells in the same inclined layer. Compared to the single depressurization method, placing a horizontal thermal injection well in the structural high of the upper inclined sandy reservoir and a horizontal production well in the structural low of the same formation improves the gas recovery efficiency nearly twice, which is recommended for production enhancement. Our findings are useful for achieving effective and economic gas recovery from natural inclined layered hydrate reservoirs.

Recognition of turbidite stages in the Massapê oil field, Recôncavo Basin - Brazil, using well logs

The integration of rock and well logs data is fundamental for the interpretation of depositional systems and sequences, a fundamental support for E&P activities. This approach was applied on the low permeability turbidite sandstones and slurries of the Maracangalha Formation, in the Massapê Oil/Gas Field, Recôncavo Basin. Well-log interpretation allowed the recognition of 23 turbidite stages, each of which records a specific period of growth of a fan system. These stages are composed of four main facies, all observed in each one of these stages: (a) fine to medium sandstones, with porosities greater than 9%, being the best reservoirs; (b) silty to very fine sandstone slurry facies, that corresponds to very muddy reservoirs with porosities less than 9%, forming important permeability barriers; (c) siltstones and (d) shales. Turbidite stages appear in the Gamma Rays logs (GR) with funnel, block and serrated log motif patterns, corresponding to dominantly thinning and fining upward signatures, recognized in several wells in the Massapê Field. The detailed correlation of these well-logs showed that these stages are present in the whole field. The mapping of high resolution turbiditic stages, considering the correlation of their four facies, will allow a better location of wells, resulting in a more efficient exploration and production development, capable of increasing the recovery of important oil reserves that still exist in the area, optimizing costs and giving greater robustness to E&P processes.

The effect of damming on biological quality according to macroinvertebrates in some Estonian streams, Central—Baltic Europe: A pilot study

AbstractThe effect of damming on the structure of the macroinvertebrate community and biological quality was studied in nine (the 3–6th order) lowland streams of Estonia, Central—Baltic ecoregion of Europe. Four habitats—reservoirs with accumulated fine sediments, reservoirs with hard bottom, and two corresponding below‐dam areas (both fast‐flowing)—were compared to study whether and how significantly the bottom substrata in dammed areas affected macroinvertebrates and biological quality downstream of dams. The standard kick‐net samples (1.25 m2, complemented with qualitative sample) were collected in autumn 2005—spring 2006. The multimetric biological quality, based on five macroinvertebrate indices (total taxa richness, EPT taxa richness, Average Score Per Taxon, Danish Stream Fauna Index, Shannon diversity) was estimated and compared with reference values. Biological quality in reservoirs with hard bottom and their downstream reaches corresponded to good, or even high quality. Conversely, damming affected biological quality significantly and negatively, above the dam if fine sediments were accumulated. The effect was the strongest within muddy reservoirs themselves (revealing moderate quality only). However, some harmful consequences of mud were observed also downstream of dams. The results also demonstrated that the indices of estimation of organic pollution and/or general quality were able to reflect significant changes in stream flow. Copyright © 2010 John Wiley & Sons, Ltd.

Stratigraphy of the Lower Cretaceous Muddy Formation and the Drainage Pattern of Its Paleovalleys, Wind River Basin, Wyoming

Valley-fill deposits of the Lower Cretaceous Muddy Sandstone contain significant stratigraphically trapped hydrocarbon accumulations and are an important exploration target in Wyoming's Bighorn, Powder River, and eastern Wind River basins. An integrated subsurface and outcrop study of the Muddy Sandstone in the western Wind River basin was conducted in order to determine (1) if Muddy paleovalleys and are present there, (2) if potential for stratigraphic traps exists, and (3) if source rocks of sufficient quality and maturity are able to charge the Muddy reservoirs. The results of the study reveal that the Muddy Sandstone (up to 100 ft thick) contains a series of unconformity bounded progradational shoreface deposits, separated by a valley-fill sequence (up to 60 ft thick). In general, the Muddy Formation is divided into three informal zones: (1) a lower marine zone, containing at least two progradational shoreface highstand parasequences; (2) a middle zone, which consists of a lowstand fluvial/estuarine valley-fill sequence (8-10 mi wide); and (3) an upper marine zone, containing at least three landward-stepping highstand shoreface parasequences. In the southwestern Wind River Basin, the Muddy paleovalley trend is north-northwest, parallel to the present-day structural trend of the Wind River thrust and its parasitic anticlines. However, insteadmore » of continuing into the potentially prospective northwestern Wind River basin, the paleovalley makes an abrupt 90{degree} turn westward, a few miles north of Ft. Washakie. A detailed understanding of how Muddy paleovalley drainage reacts to basement faults could become a key predictive tool for future Muddy exploration in the northern Rocky Mountains.« less

Integrated Geologic and Geophysical Mapping: An Example from the Muddy Sandstone Wind River Basin, Wyoming: ABSTRACT

Muddy Sandstone stratigraphic traps have been the primary stratigraphic exploration target in the Wind River basin (WRB) for BP Exploration and many in industry since the discovery of the Grieve field (30 MMBO, 60 bcf of gas) in 1954. Since 1984 in the eastern WRB, 18 wildcats have been drilled resulting in three new field discoveries which total approximately 23 MMBOE. BP Exploration staff has successfully predicted the reservoir outcome of over 95% of the wildcats by incorporating log facies and facies models with seismic amplitude mapping of the Muddy reflector. The Early Cretaceous Muddy Sandstone unconformably overlies the earlier Cretaceous marine Thermopolis Shale and is conformably overlain by the marine Mowry Shale. An incised valley system was cut into the Thermopolis Shale during a eustatic sea level drop. During the ensuing sea level rise in the southeast portion of the WRB, the Recluse sandstone is the primary Muddy reservoir unit. Secondary Muddy sandstone deposition of marine shoreline sandstones occurred during fluctuations of the primary eustatic sea level cycle. Therefore, in certain areas of the WRB , the Recluse member sandstones can be overlain and/or underlain by marine sandstones. In areas where the older and younger marine sandstones are absent,more » seismic amplitude mapping, incorporating well control, provides a method of mapping the extent of the valley fill sediments. The Muddy valley-fill sediments are generally less than 30 m thick and are encased in slower velocity shales. When valley-fill sediments are present, the Muddy reflector is high amplitude in contrast to low amplitudes when the valley-fill is absent. Estimates of valley fill lithology are provided through synthetic modeling of well logs representing different lithofacies. The resulting wavelet characterization, which includes wavelet shape and peak-to-trough time separation, improves predictability of sandstone distribution.« less

Geologic and Seismic Models, Muddy Sandstone, Lower Cretaceous, Bell Creek-Rocky Point Area, Powder River Basin, Montana and Wyoming: ABSTRACT

Abstract The Muddy Sandstone in the northern Powder River Basin consists of two distinct members (genetic units) that are separated by a widespread subaerial surface of erosion. The older sandstone member, the reservoir rock at the giant Bell Creek field, was deposited in shoreline and associated nearshore marine environments. It is genetically related by intertonguing to the underlying Skull Creek Shale. The younger member is a valley fill deposit of fluvial, estuarine and tidal flat environments. The deposition and distribution of these two genetic units were controlled primarily by relative sea level changes. The Bell Creek sandstone was deposited as a widespread regressive sandstone during a highstand of sea level. A following sea level lowstand caused valley cutting, erosion of all or portions of the Bell Creek sandstone, and paleosoil development causing early diagenesis, especially of kaolinite, siderite and quartz. A rising sea level resulted in valley filling, coastal onlap and a transgressive surface of erosion that is overlain by black marine shale (Nefsy or Shell Creek). Recurrent movement on basement-controlled fault blocks appears to have controlled distribution of the Muddy members, drainage incisement patterns, present structure and heat flow, and possibly petroleum migration. Seismic modeling indicates that it may be possible to recognize seismically the distribution of Muddy reservoir facies that occur in the area. Seismic modeling suggests that the amplitude response of the Muddy is influenced by Muddy facies and thickness changes. The lowest Muddy amplitude observed occurs when the Bell Creek sandstone is prevalent. An increase in the Muddy amplitude is noted when thick valley fill deposits are present. Results from seismic modeling compare well to actual seismic data. Amplitude changes noted in the Muddy reflector in the Bell Creek – Rocky Point area may help explorationists in the future to distinguish between nonprospective and reservoir quality Muddy Sandstone.

Hydrodynamic Flow in Lower Cretaceous Muddy Sandstone, Gas Draw Field, Powder River Basin, Wyoming

Lower Cretaceous Muddy sandstones form a simple stratigraphic trap at Gas Draw field, northeast Powder River Basin. The Muddy at Gas Draw can be subdivided into six zones. The lowest, sixth Muddy sandstone is, fluvial in origin, and the overlying fifth sandstone is a transgressive marine deposit. The fourth zone represents a fluvial origin below to shallow marine above. The third zone is interpreted to be of fluvial-overbank origin with possibly a bay-lagoonal association. The second sandstone is suggested to be a deposit of a fluvial or deltaic environment. The first zone was deposited in a lagoonal and poorly-drained marsh environment. At Gas Draw, the second Muddy sandstone is the major producing zone and has the highest average porosity and permeability of 22.6% and 209 md. It had high initial production rates of up to 1200 BOPD (191 m3/day). From correlation of well logs, the Muddy reservoirs appear to be separated by thin shales, but analysis of drill stem tests show that these reservoirs are connected at some points within the area. Analysis of drill-stem test pressures provides the basis for interpreting fluid potential relationships. The average potentiometric gradient of 32 ft/mi across the field results in a hydrodynamic oil column of 210 feet, whereas capillary-pressure differences due to permeability changes can account for only 38 feet of oil column. The ob­ served oil column over most of the field has a height somewhat greater than 250 feet which agrees well with total calculated oil column of about 248 feet. Furthermore , local decrease in permeability to oil may be responsible for water production at any place within the field, even updip from the producing area. Therefore, the trapping capacity of this stratigraphic trap is not simple but depends on rock properties and fluid-pressure relationships within the reservoir .

Downward Vertical Fluid Flow in Subsurface: Implications at Kitty Field, Powder River Basin, Wyoming: ABSTRACT

Drill-stem test pressure data and subsurface geologic studies permit interpretation of fluid-potential relations in the Kitty field area of the Powder River basin. These relations provide substantiation of significant downward fluid flow from Mowry Shale source rocks to Muddy reservoirs in areas where maximum thicknesses of permeable sandstone are developed. Potentiometric contours indicate vertical flow becomes lateral and radiates outward from areas of high potential once confined to the Muddy aquifer system. Pressure distribution within individual reservoir beds is consistent with this interpretation. Vertical flow is presumably sustained by a combination of the following: (1) expulsion of bonded water during thermal alteration of mixed-layer clays in the Mowry Shale; 2) water generation associated with catagenesis and oil generation in the Mowry Shale; and (3) aquathermal pressuring at temperatures in excess of 200°F (94°C). Local downward vertical flow of water at Kitty field may provide the means for hydrocarbon migration from Mowry source beds to Muddy reservoirs. Additionally, pressure gradients associated with this flow may be responsible for trapping approximately 75% of the total oil column in the field. Capillary pressures can account for a maximum of about 200 ft (61 m) of the total observed 835 ft (255 m) oil column. Calculations suggest that an additional 460 ft (140 m) of oil represents the minimum hydrodynamic column and that fluid-potential gradients necessary to trap the additional 175 ft (53 m) probably exist in the field. The Muddy potentiometric surface implies a dynamic aquifer system of downdip inter-formational and vertical, cross-formational fluid flow. Early fluid migration is obscured by this present flow, but can be pressured to be updip toward the basin flanks in response to initial compaction processes. Post-Laramide (Eocene?) exposure of the Muddy aquifer resulted in recharge by meteoric waters. The interaction between meteoric downdip regional flow and local vertically downward flow at Kitty field suggests late accumulation of hydrocarbons. End_of_Article - Last_Page 947------------

Muddy Formation of Northern Powder River Basin--A Stratigraphic Paradox: ABSTRACT

The Muddy Formation in the northern Powder River basin contains a stratigraphic paradox whereby the oldest basal sandstones appear to be the youngest and vice versa. Subsidence history is the underlying cause of the paradox. Relating depositional events as determined from physical and paleontologic data to the subsidence leads to a simple and consistent depositional history. On lithology, the Muddy is subdivided into lower and upper units. The lower unit consists of thick basal sandstones and thin contemporaneous siltstones and shales that were deposited by a single southeast-to-northwest regression. These thick sandstones are important Muddy reservoirs and consist of coastal barriers (Bell Creek and Rozet fields) and distributary or estuarine channels (Recluse field). When the lower Muddy regression had proceeded to a point northwest of the currently producing area, the character of the shoreline changed from a high-energy sandy shoreline to a low-energy muddy and tidal-flat shoreline. The shoreline remained northwest of the productive area throughout the rest of Muddy deposition, End_Page 618------------------------------ although subsidence was greater there than in the southeast. This differential subsidence caused the southeast thinning of the nonmarine upper Muddy sediments. Muddy deposition was terminated by the rapid eastward advance of the Shell Creek or lower Mowry sea. This transgression reworked the uppermost nonmarine Muddy sediments, producing a thin, widespread sandy zone that constitutes the principal reservoir at Hilight field. Because the lower Muddy was deposited during a southeast to northwest regression, the sandstones are progressively younger toward the northwest. However, the overlying nonmarine upper Muddy thickens northwestward, causing the underlying regressive sandstone to appear to be stratigraphically lower and, hence, older in that direction. This stratigraphic paradox has caused much confusion and difficulty in resolving Muddy stratigraphy. End_of_Article - Last_Page 619------------

Microbiologic Alteration of Crude Oil in Muddy Sandstone and Other Reservoirs: ABSTRACT

Loss of paraffin wax constituents from crude oil has been attributed by some investigators to microbiologic activity in the reservoir, but until now no significant evidence supporting this hypothesis has been reported. Gas chromatographic and mass spectral data now indicate that a highly selective loss of n-paraffins has occurred in some oils. A classic example of selective n-paraffin removal is found in the Cretaceous Muddy reservoir at Bell Creek field, Montana. Occurring within the same reservoir system are (1) oil showing no loss of n-paraffins, (2) oil with all n-paraffins missing up to about n-C17, but unaffected above n-C17, and (3) oil with the entire range of n-paraffins missing. A microbiologic process seems to be the only logical explanati n for such high selectivity. Surface recharge waters apparently transport aerobic microorganisms along with the necessary oxygen to accessible reservoirs. Other evidence for microbial activity at Bell Creek includes increase in nitrogen content and optical rotation disproportionate to n-paraffin loss, suggestive of addition of microbially produced materials to an altered oil. Optical rotation, carbon isotope, and mass spectral data obtained on distillation fractions of Muddy oil from Whitetail field, near Bell Creek, provide direct evidence that components are being added to the oil by microbial activity. In East and West Hackberry fields, Louisiana, there is evidence that some branched as well as straight-chain hydrocarbons have been microbially destroyed. Examples of microbial alterati n have been found in most oil-producing areas of the world. End_of_Article - Last_Page 369------------

Depositional Environments of Muddy Reservoir Sandstones (Lower Cretaceous) in Powder River Basin, Montana and Wyoming: ABSTRACT

Lower Cretaceous Muddy Sandstone forms stratigraphic traps in the northeast Powder River basin. The most productive reservoirs are local concentrations of clean, quartzose sandstones deposited in linear bodies as marine bars. Sediment supply to the bars was accomplished by transverse sedimentation from associated fluviodeltaic environments. Two distinct marine bar types may be recognized: (1) regressive, interdeltaic barrier bars, and (2) transgressive, delta-destructional bars. In regressive bars, grain size increases upward, and subenvironments which may be recognized include, in ascending order, lower shoreface, middle shoreface, upper shoreface-beach, and eolian. In transgressive bars, the vertical sequence of grain size and subenvironments is reversed with grain size decreasing upward, and subenvironments generally commencing with middle shoreface and succeeded upward by lower shoreface. Both bar varieties have prominent lagoonal or tidal-flat deposits as lateral equivalents. These fine-grained, back-bar sediments act as effective barriers to updip petroleum migration. Transgressive bars are associated closely with typical fluviodeltaic deposits which they may succeed, laterally or vertically, in close succession. Therefore, they are interpreted as delta-destructional bars. Regressive bars, however, are laterally and vertically removed from the main sites of delta construction, and are interpreted to be typical coastal, interdeltaic deposits. Although fluviodeltaic sandstones are local sources for bar sands, they are mineralogically quite distinct. Significant quantities of rock fragments and clay-rich matrix reduce the relative quartz content. Therefore, porosity and permeability values are low and, despite the fact that the fluviodeltaic sandstones are locally thick, they form less productive reservoirs. End_of_Article - Last_Page 835------------