engleski

Some Possibility of SP Logs in Resolving Oil Field Geological Model

The southern margin of the Pannonian Basin in Croatia includes around thirty small- to medium-size oil and gas fields. They are located along two depressions that strike along the Drava and the Sava river-valley, respectively. There, we may clearly differentiate two rock complexes in both stratigraphic and lithological terms. The older pertains to the Paleozoic, and, to a lesser extent, to the Mesozoic. The oldest rocks are granites and gneisses, followed by various types of metamorphic, magmatic and sedimentary rocks. Depending on the paleorelief conditions and related paleogeological relations, this complex is unconformably overlain by the younger, Upper Tertiary sedimentary complex, beginning with transgressive and lithologically variable Lower Miocene rocks, covered by the Pannonian and Pontian marls and sandstones, Pliocene clays and sands, and finally, by the Quaternary deposits. Some oil reservoirs are in the basement rocks, but the largest part is in the Upper Pannonian and Lower Pontian, which is characterized by the alternation of sandy and marly beds. The reservoir rocks are predominantly fine- to medium-grained quartz-mica sandstones, occasionally finely laminated. Sporadically, they clearly show textures typically related to channel beds, while those deposited on a slightly uneven basement, with relatively small recesses, suggest a frequent change in material transport conditions regarding the type, volume, and transport direction. Such sedimentary conditions have caused abrupt changes of the petrophysical properties within the reservoir rocks, both horizontally and vertically. These changes are reflected in frequent transitions from good porosity and permeability zones of sandstones and silty sandstones, to poorly permeable or nearly impermeable zones of marly sandstones. Such composition is the major cause for considerable differences among the oil pools' effective thicknesses, permeability disruptions, and hydrocarbon saturation distribution. Described lack of uniformity has considerable influence to oil production, particularly on those fields or in reservoirs where it is performed by the water injection into the reservoirs, for the purpose of pressure maintenance. The monitoring of pressures, as well as fluid types and volumes, has shown that some parts of the field do not respond at all to water injection. Sometimes the pressure in the reservoir drops and the oil supply volume goes down with only a small water share. On the other hand, in some parts of the reservoir, the pressure is maintained at the expected level, but the fluid supply goes up, accompanied by oil amount lowering and water share increase. Due to such differences in reservoir properties, it is necessary to determine the areas with the same i.e. similar properties to achieve a more efficient and economical draining. This regards not only the fields that have already been put on stream, but particularly those that are being developed, where the problem has not appeared yet, but may well be expected. The isolation and analysis of the zones has been performed on the fields in the Drava depression (e.g. Bilogora, Legrad...) and in the Sava depression (Ivanić, Okoli...). Some of them clearly show zone contours that are changing rhythmically, while, in some other cases, zones are grouped with no apparent pattern. The fields that we should like to mention here in particular are Žutica and Kloštar (the Sava depression), and Šandrovac (the Drava depression). They have been developed by a sufficient number of wells, however, during their development, the problem in question was not that much present. During the completion of the production wells, there was no sufficient sampling as was the case in the exploration phase. The analytical procedures necessary for determining the sedimentation system were either not performed, or, if so, they were not adequate. However, there is a number of conventional e.-logs (spontaneous potential, SP, apparent resistivity, Ra), as well as a microlog of all wells. The rest of the electric logging was mostly not performed on them. That is why, based on the similarity of SP curve forms, we have tried to group the wells into similar, connected zones. The zones' distribution is shown on the maps, while their vertical properties are presented on profiles. This could be a variant of electrofacies maps. In order to determine the similarity, we have used the following basic elements: curve shape, layer thickness, the number of "pure" sandy beds. In cases where logs showed sufficient similarity according to these criteria, and providing that the wells were not too far apart or separated by wells with other properties, we were able to isolate a special zone by comparing the SP log with the SP log of each of the nearest neighbouring wells. FIGURE 1. Every zone was assigned a special mark in order to enable comparison between mutually distant parts of the same reservoir. By introducing the data on petrophysical parameters, pressure, and volumes and type of the fluids drained at particular wells into the zone map, we have come up with satisfactory results, since the same i.e. very similar data were grouped within the individual zones' surfaces. The Žutica field is located in the central part of the Sava depression. There is no evidence of any serious tectonic disturbance. The reservoirs are in anticlinal traps, the major anticline axis trend is northwest to southeast. The main oil production object, the gamma series, of the Upper Pannonian age, is thoroughly described here. Based on mineralogical analyses, the reservoir rocks are quartz-mica sandstones, of unhomogenous composition and granulation. They may be more silty or containing marl intercalations. It could be separated in 8 beds, ranging in thickness from 8 to 50 m. Their acreage also varies, as well as their distribution. Alternating with sandstone beds, there are poorly permeable to impermeable silty sandstones and marls, also showing variable thickness (2-15 m). According to the described procedure, only several zones have been separated, with their regular contours strike parallel to the major anticline axis, thus pointing to the material transport directions (Bokor and Vujnović, 1991). If we look, for instance, at the 1B zone (Fig. 1) with its corresponding representative SP log, we can see that the average reservoir thickness amounts to 20 m with only small variations within the same zone. The reservoir has a good permeability. The porosity ranges from 0.234 to 0.249, and the permeability from 31.6-47.2x10-3 mm2. During the entire production, the oil supply was not disturbed. In the neighbouring 2A zone, the reservoir thickness is nearly the same. The porosity is somewhat weaker, ranging from 0.192 to 0.206, and the permeability from 22.1 and 23.8 x10-3 mm2. FIGURE 2. Hydrodynamic explorations have revealed that the links between the wells pertaining to different zones are either poor or even virtually nonexistent. Differences in production are undoubtedly the result of local lithological changes. Given this, all further works on the recovery of remaining hydrocarbon reserves were co-ordinated according to the SP logs similarity zones distribution, which yielded satisfactory results. The Šandrovac oil field is located in the southern part of the Drava depression. It is a brachyanticline dissected by numerous faults. The general fault strike orientation is north-south, perpendicular to major brachianticline axis with general throw between ten and several tens of meters. There are several reservoirs, but the main are represented by the E series, i.e. E, E', E", E"'. All are of the Lower Pontian age. These are beds with a characteristic fossil shell Paradacna abichi. According to their composition the sandstones are similar to those on Žutica oil field containing quartz and platy minerals, as well as plagioclases. They are fine- to medium-grained and non-cohesive to poorly cohesive rocks interbedded with marls. Thickness of reservoir sandstone beds range between 5-25 m. Due to the sedimentary conditions, they show frequent qualitative changes in reservoir properties, both in horizontal and vertical direction. By the analysis of SP curve forms, we have separated numerous smaller zones with different contour forms and irregular distribution (Fig. 2), but consistent with the data obtained during the oil production. Similarly to other cases, electrofacies analysis (Reading, 1978, Cant, 1984) was also performed and several zones are separated, but their boundaries almost perfectly match to the series of zones determined through the comparison of SP logs. The Kloštar oil field has certain similar properties. It is located to the north of Žutica. It is an elongated anticline striking from northwest to southeast, disturbed by faults. The Miocene sediments unconformably overlie the basement composed of granite and gneiss. The oil reservoirs occupy a wide vertical range, starting at the basement through the Badenian, Sarmatian, Pannonian up to the Lower Pontian. The alpha and beta reservoirs of the so-called 2nd sandy series, corresponding to the gamma series on Žutica, were particularly analyzed. The reservoirs are composed of fine- to medium-grained sandstones. They are situated on the southwestern part of the field. The reservoir thickness is mostly uniform (2-15 m), with only small variations, and gradually increases towards the southwest. Both reservoirs are pinching out towards the central part of the field, with the alpha reservoir having a smaller acreage than the beta one. Both are characterized by the primary-stratigraphic trap. The zones determined by the comparison of SP log forms have relatively regular contours and to a certain extent follow the sandstone distribution boundary. These contours mostly correspond with the zonal contours of electrofacies and physical parameters. The pointed SP log forms indicate the margin of the sandstone sedimentary body. A systemathical analysis of SP log forms has shown that, even in the lack of a sufficient number of cores i.e. lab analyses, it is possible to separate zones of similar lithological properties and physical parameters, and hence to determine the geological model, which bears an impact on the optimum oil yield. REFERENCES: Bokor, N. and Vujnović, S. (1991): Sedimentation Models and their Role in Yield Increase, DIT, INA-Naftaplin, 29-30/IX, pp. 55-62, Zagreb. Cant, D.J. (1984): Subsurface Facies Analysis. -in R.G. Walker ed.: Facies Models. Geoscience Canada Reprint Series 1, Geol. assoc. of Canada, 297-310. Reading, H.G., ed. (1978): Sedimentary Environments and Facies. Blackwell Scient. Publ., 557.

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