John Kaldi

Geological applications of capillary pressure: A review

Capillary pressure concepts can be used to evaluate reservoir rock quality, expected reservoir fluid saturations and depths of fluid contacts, thickness of transition zone, seal capacity, and pay versus nonpay, and to approximate recovery efficiency. Mercury-injection capillary pressure is typically favored for geological applications, such as inferring the size and sorting of pore throats. The differences between mercury injection and withdrawal curves can provide information on recovery efficiency. The height above free water level can be determined by comparing capillary pressure data to hydrocarbon shows and measured fluid saturations. Capillary pressure data can also be used to distinguish reservoir from nonreservoir rocks and pay from nonpay on the basis of nonwetti g-phase saturations. Other applications of capillary pressure data include relating capillary pressure to absolute and relative permeabilities, and using porosimetry to investigate pore-level heterogeneity. This paper reviews geological applications and interpretation of capillary pressure in reservoir studies.

Early diagenetic dolomite cements; examples from the Permian Lower Magnesian Limestone of England and the Pleistocene carbonates of the Bahamas

ABSTRACT Early diagenetic dolomite cements are found in the Permian Lower Magnesian Limestone of eastern England and the Pleistocene carbonates of Great Abaco Island, Bahamas. In the Lower Magnesian Limestone the dolospar occurs as isopachous rinds that have grown both centripetally and centrifugally on the dolomitized micrite envelopes of hollow-centered allochems. Because compactional fractures penetrate both the allochem walls and the cement rinds, dolomite cementation is inferred to have predated any significant sediment accumulation. In the Great Abaco carbonates, limpid dolomite forms isopachous rinds around inter- and intragranular porosity, and is postdated by meteoric calcite cement. Both examples of early diagenetic dolomite cement textures developed as a consequence of changes in pore-fluid chemistry from marine, to mixed meteoric-marine, to meteoric waters. Therefore, the presence of this cement type may point toward aliagenetic environments resulting from marine regressions.

Pore Geometry: Control on Reservoir Properties, Walker Creek Field, Columbia and Lafayette Counties, Arkansas

Walker Creek field in southern Arkansas produces hydrocarbons from oolitic packstones and grainstones of the Jurassic Smackover Formation. The relationships between pore geometry and reservoir quality in these rocks were evaluated using petrographic methods and mercury injection capillary pressure analyses. Results indicate that reservoir quality is controlled by pore geometry, which, in turn, is determined by depositional and diagenetic processes. Reservoir rocks at Walker Creek were deposited as prograding grainstone shoals in a shallow-water, high-energy environment. Diagenetic processes, including early marine cementation, compaction, and deeper burial pressure solution and calcite cementation, modified the original pore system. Primary interparticle porosity is the dominant effective pore type and is most important in terms of reservoir performance. Secondary microporosity is also abundant, comprising a significant percentage of total porosity (locally up to 100%), but is generally ineffective. The rocks were subdivided into lithofacies based on their depositional and diagenetic origin. Within each lithofacies, reservoir, marginal reservoir, and nonreservoir rock types are identified based on their pore geometry and characteristic capillary pressure curve. The reservoir rock types are less cemented oolitic and oolitic-skeletal grainstones with steeply sloping capillary pressure curves and flat plateaus indicating relatively homogeneous pore sizes that are well interconnected by large pore throats. The marginal to nonreservoir rock types include mudstones, wackestones, muddy packstones, and cemented grainstones with generally small interparticle and moldic pores. Gently sloping capillary pressure curves with poorly defined or no plateaus indicate variable pore and pore-throat sizes. A close correlation is found among parts of the capillary pressure curve shape, such as slopes, plateaus, and displacement pressure, and aspects of pore geometry such as pore-throat size, pore-throat size distribution, and pore interconnectivity. In addition, capillary pressure analyses provide guidance in evaluating overall reservoir quality by rock type, as well as answer specific questions regarding net pay criteria as a function of height above free-water level.

Diagenetic microporosity (chalky porosity), Middle Devonian Kee Scarp reef complex, Norman Wells, Northwest Territories, Canada

Abstract Diagenetic microporosity (chalky porosity) is essential to reservoir development in the Norman Wells oilfield. The pore system consists of intercrystalline voids within a micro-rhombic (2–10 μm rhombs) low-magnesium calcite matrix. Microporosity formed early in the diagenetic history of the Kee Scarp Formation, possibly prior to deposition of much of the overlying Canol Shales. Petrographic evidence suggests that the microporous texture developed as a three-stage recrystallization phenomenon (incipient, immature, and mature microporosity) from contemporaneous mixed marine/meteoric fluids. Microporous reservoir rocks occur mainly in stratigraphic horizons that are above the platformal limestone of the reef complex, and preferentially within reef margin and foreslope carbonate sand facies. Microporosity is more common in fine-grained matrix than in macrofossils, however, where fossils have undergone “chalkification”, cylindrical/branching, lamellar, and massive stromatoporoids are affected preferentially. Extensive fracturing and at least two stages of calcite cementation postdate the formation of microporosity; fractures increase deliverability while calcite cements diminish reservoir potential.

Diagenesis of nearshore carbonate rocks in the Sprotbrough Member of the Cadeby (Magnesian Limestone) Formation (Upper Permian) of eastern England

Summary The petrographic textures in the upper member of the Cadeby (Magnesian Limestone) Formation demonstrate the combined effects of original depositional setting and later diagenetic alteration of a carbonate rock. Dolomitization was the most ubiquitous diagenetic process to affect these rocks. Dolomites formed (1) penecontemporaneously, (2) perhaps as primary precipitates, (3) as cements during early diagenesis, and (4) as locally controlled late diagenetic replacement and cement. These dolomite textures were subsequently altered by the advent of new diagenetic environments. A drop in Zechstein sea-level exposed much of the upper (Sprotbrough) member to the influence of meteoric waters. The mixing of fresh and marine waters produced solutions capable of dissolving unstable carbonate matter, dolomitizing more resistant grains and replacing earlier dolomite textures with new ones. Generally, later dolomitization events produced coarser crystal textures than the earlier. Evaporite minerals were formed as early nodules within supratidal sediments or as late diagenetic pore-filling cements in the sub-surface. Their dissolution led to the development of considerable vuggy porosity and the addition of Ca2+ to ground waters. Ca2+-enriched fluids calcitized surrounding dolomites to produce dedolomite. Porosity was controlled by the changes from one diagenetic environment to another. Primary pores were modified by cementation, recrystallization and pressure-solution. Secondary porosity was created by the dissolution of minerals that were unstable in pore fluids of the later diagenetic episodes.

Sedimentology of sandwaves in an oolite shoal complex in the Cadeby (Magnesian Limestone) Formation (Upper Permian) of eastern England

Summary Four depositional facies are recognizable in the Sprotbrough Member of the Cadeby Formation (formerly the Lower Magnesian Limestone) in eastern England; they are sabkha, semi-restricted lagoon, oolite shoal complex and open marine shelf. Of these, the oolite shoal complex occupies most of the present outcrop in the Yorkshire Province, i.e. south of the Cleveland High. It comprises large-scale cross-stratified oolite bedforms that are interpreted as sandwaves from their size, type of sediment and internal features. The sandwaves probably originated from the concentration of wave energy by eminences on the floor of the Zechstein Sea. Biogenic structures in the sandwaves show that they were inhabited by marine organisms. A complex pattern of palaeocurrent styles and azimuths is present within the sandwaves. At many exposures, the long axes of the sandwaves trend NW-SE, suggesting predominant currents normal to this direction. A predominant current from the NE is further indicated by the preferential abutment of SW-dipping beds on the NE flanks of bedforms; this was caused by sandwaves that climbed the backs of others. Occasional storms from the SE produced spillover lobes oriented towards the NW. Storms were also probably responsible for the formation of internal structures such as hummocky cross-stratification, fan-shaped bedding and contemporaneous erosion surfaces.

CO2 storage capacity estimation for a deep saline formation by static reservoir modelling: the Cretaceous Paaratte Formation, offshore Otway Basin, Victoria

The APPEA Journal is multidisciplinary technical journal documents peer-reviewed papers presented at APPEA’s Annual Conference. From 2008 onward the APPEA Journal is available in DVD format only.

Petrological Reconstruction of the Subsurface Based on PDC Drill Cuttings: An Advanced Rock Typing Approach

Recent advances in automated scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS) have transformed the petrological analysis of drill cuttings by replacing qualitative descriptions with ultra-fast, quantitative and repeatable analysis. In this study, 100 m of drill cuttings produced by Polycrystalline Diamond Compact (PDC) bits were mapped on a cutting-by-cutting basis, providing information on mineralogy and texture. The individual cuttings were classified into lithotype categories based on mineral composition and grain sizes. These lithotyping data complement bulk sample mineral and chemical assays, providing detailed information on the nature of cementation, intra-formational seals and changes in mineral grain sizes between reservoir sections. The petrological reconstruction generated from the cuttings analysis compares well with the gamma-ray well log. By combining automated SEM-EDS measurements with advanced digital image analysis, meaningful geological information can be generated from PDC drill cuttings, improving our understanding of lithostratigraphic subdivision and zonation.

Depositional Environments and Diagenesis of Mississippian Midale Beds, Midale Field Area, Williston Basin, Southeastern Saskatchewan: ABSTRACT

The Midale oil field in southeastern Saskatchewan lies on the northeastern flank of the Williston basin. Oil occurs mainly in Mississippian strata that dip south-southwestward and are truncated progressively northward by a Late Mississippian-Early Jurassic erosion surface. The reservoir is in the Midale beds, a suite of carbonates and evaporites that was deposited during several transgressive-regressive episodes in a shallow shelf environment. The Midale beds produce predominately from the Midale carbonate, which is divided into three zones. The lower zone represents a restricted, possibly lagoonal environment in which moderate energy conditions occurred intermittently; the middle zone formed in a transgressive, moderate to high-energy shoal environment; and the upper zone carbonate originated in restricted subtidal conditions. Oil reservoirs are coarsely crystalline vuggy dolomite and fractured, bioturbated calcareous dolomite of the middle and upper zones, respectively. Diagenesis resulted in the formation of various stratigraphic traps. Early syntaxially cemented crinoid banks form local reservoirs. Field-wide leached intercrystalline porosity and microfractures are the economically most significant porosity types. Based on the knowledge of local depositional environments, diagenesis, structural contours, and isopach maps, it is possible to high grade reservoir predictability. End_of_Article - Last_Page 852------------

Porosity Styles of the Midale Field in Williston Basin of Southeastern Saskatchewan: ABSTRACT

The Midale oil field, southeastern Saskatchewan, lies on the northeastern flank of the Williston basin. The reservoir is in limestones, dolomites, and evaporites (mainly anhydrite) of the Midale Beds (Mississippian), that were deposited during a predominantly regressive episode on a shallow shelf. The Midale Beds are divided into a lower, middle, and upper zone. Many of the characteristic pore types in these zones, as observed in thin section and under SEM, can be related to both original depositional environment and postdepositional diagenetic modifications. The dominant styles of porosity in the fine-grained argillaceous limestones of the lower zone are secondary intraparticle, moldic, or microvuggy. These fabrics result from the preferential dissolution of cement or very fine shell debris. A lack of pore interconnections precludes these sediments from being effective reservoir rocks. The most significant pore type in the middle zone is secondary intercrystalline porosity within fine-grained dolomite. This fabric is the result of solution of calcite or aragonite from between rhombs after incomplete dolomitization. A crinoidal grainstone with pervasive early diagenetic syntaxial rim cement forms a tight trap in the middle zone. The upper zone consists of fractured calcareous microcrystalline dolomite. The main pore type is a non-fabric selective system of oblique to vertical microfractures, which may be associated with regional uplift or local salt solution. The presence of dolomite rhombs on fracture surfaces indicates that dolomitization was relatively late, postdating fracturing. The microfractures in the upper zone counteract the porosity-occluding effects of stylolitization and secondary anhydritization. End_of_Article - Last_Page 587------------