Класс Месторождения:
Тип Месторождения: Газоконденсатное
Местоположение:
Местность:
Стадия разработки: Добыча
Год открытия: 1981
Источник информации:
Метод открытия:
Площадь: 57.07 км²
Sleipner Øst (SLØ)
Sleipner Øst (SLØ) was discovered in Block NO15/9 by well 15/9-9 in 1981 (Østvedt, 1987), and development commenced in 1993 (Figure 1).
FIGURE 1. Top Ty formation depth map with the location of the production platform (SLA), wells, and the pinch-out of the formation in the southeast area. Contours are true vertical depth in meters (20-m [66-ft] interval) below the mean sea level (TVDMSL). Wells referred to in this study are marked with a large bold font, and the position of the cross sections (AA0 and BB0) in Figure 3 are highlighted.
The block is situated within the southern Viking Graben and is operated by Statoil with their license partners Esso, Norsk Hydro, and Total. The hydrocarbon charge is gas condensate in Triassic, Jurassic, and Tertiary reservoirs with the Tertiary (lower Paleocene) Ty formation reservoir the focus of the chapter (Figure 2).
FIGURE 2. Paleocene stratigraphy in the North Sea. The Ty formation in the Viking Graben, which is laterally equivalent to the Maureen Formation in the central trough, overlies the Ekofisk Chalk and is coeval with the Va˚ le shales (modified from Isaksen and Tonstad, 1989).
Six exploration and appraisal wells were initially drilled; since then, 13 production and five dry-gas injection wells have been drilled in the Ty formation reservoir (Figure 1). In addition, five wells have penetrated the Ty formation to investigate deeper targets. By 2011, SLØ is expected to have produced 78% of its reserves, and it is recognized as a potential site for future gas storage from other fields in the area. In the context of maximizing condensate recovery, optimizing late field-life production, and subsequent amenability to use for gas storage, it is important to assess the effect of mud-prone intervals on the sweep efficiency of the reservoir.
The Ty formation was deposited in a deep-marine setting and has excellent reservoir characteristics (Table 1). Sand is interpreted to have been sourced from the East Shetland platform (Isaksen and Tonstad, 1989; Galloway et al., 1993; Strømmen et al., 1998), transported toward the east by gravity currents with rapid deposition in the Sleipner area induced by irregular sea-floor topography. In the SLØ area, the Ty formation is sand rich, and a rapid change in thickness is present from approximately 150 m (492 ft) in the northwest to zero in the southeast, where the sandstone pinches out (Figure 1). Using high-resolution biostratigraphy of the mudstones, nine reservoir zones can be defined. The mud-prone units are as much as 5 m (16 ft) thick and can be correlated throughout most of the SLØ area (Figures 3, 4). Less frequently, thinner mudstone intervals occur within otherwise sand-rich layers but cannot be correlated between wells.
FIGURE 3. Cross sections through the Ty reservoir (see Figure 1 for location). The correlation of the nine reservoir zones and the major shale (hemipelagic) units between the wells is illustrated. These shale units are illustrated as continuous between wells; however, production data show the shalestones to be discontinuous, most probably dissected by sand injectites, and possibly by subseismic faults. Depth in meters true vertical depth below sea surface (m TVDSS).
FIGURE 4. Wire-line logs from well 15/9-A-26. The low gamma-ray (GR) response and the yellow color fill between the neutron porosity (NPHI) and bulk density (RHOB) logs record sandstone. The high GR reading and the green color fill between the neutron porosity and bulk density logs record mud-prone units. Postproduction repeat formation tester (RFT) data points record the reservoir pressure (bars) at specific sampling points in the well. The data show vertical pressure communication in the well, with all reservoir zones equally depleted by production. Blue arrows locating the sandstone injectites are observed within core. CALI = caliper; SW = water saturation; PHIF = porosity; KLOGH = permeability (md); RT = resistivity; DT = sonic; MD = depth below the platform measured along the well path; and TVDMSL = true vertical depth relative to mean sea level.
Recent cores taken from development wells have highlighted the abundance of centimeter-scale injected sands in mud-prone intervals. These include sandstone sills and dikes (Figure 5A) that are commonly distorted by compaction, dikes that incorporate mudstone clasts from the surrounding depositional mudstones (Figure 5B), and sandstones that inject into and deform depositional sandstone units (Figure 5C).
FIGURE 5. Examples of microscale sandstone injections present in the Ty formation. s = sandstone and m = mudstone; white bar is 5 cm (2 in.) long, and white dashed line indicates depositional bedding planes. (A) Ptygmatic folded sandstone dike (black arrow) propagating upward from a sill (white arrow). Note the sand-filled microfracturing in the mudstone. (B) Sandstone dike (s1) with a high concentration of millimeter- to centimeter-scale mudstone clasts. (C) Sandstone dike (s1) injected into a depositional sandstone unit (s), with an offshooting sill (black arrow) starting to develop as further upward injection of sand was hindered by a mudstone unit. Depth shale (m MD) refers to depth below platform measures along the wellbore.
Similar features seen in the more sand-prone intervals that were previously interpreted to contain predominantly primary depositional structures and secondary structures produced by liquefaction may alternatively be considered as injectites in the light of these observations (Hurst and Cronin, 2001).
DATA
Cores from four wells (15/9-A-8, 15/9-A-22, 15/9- A-26, and 15/9-D-3 H) are presented to highlight the range in sand-injection structures in the Ty formation reservoir from the SLØ area.
Well 15/9-A-26
A unit of slightly disrupted green-gray mudstones is cut by several fine- to medium-grained centimeterscale sandstone dikes, above which a 5.5-m (18.0 ft) unit of medium-grained sandstone is dominated by dish structures and consolidation laminae (Figure 6).
FIGURE 6. Well 15/9-A-26. (A) A sandstone unit with dish structures and consolidation laminae overlies a mudstone permeated by sand-filled microfractures. (B) At the sandstone-mudstone boundary, a sandstone dike with incorporated mudstone clasts has intruded. Vertical progression of the dike appears to have been hindered by the sandstone probably because consolidation of the sandstone by liquefaction has reduced the vertical permeability. The location of the interval is shown in Figure 3. Scale bars are 5 cm (2 in.) long, and bedding is near horizontal in the core. Depth shale (m MD) refers to depth below platform measures along the wellbore.
At the base of the medium-grained sandstone (Figure 6B), a 3-cm (1.2-in.)-thick, fine-grained sandstone, which incorporates slivers of green mudstone, is present. The sandstone and mudstone appear slightly sheared and are interpreted as a small intrusion that is inferred to be related to the dikes seen in the underlying mudstones.
Above the main reservoir interval, a 6-m (19-ft)- thick sandstone unit occurs (Figure 3, 2301 –2307 m [7549 –7568 ft] true vertical depth relative to mean sea level [TVDMSL]), which overlies a 4-m (13-ft)-thick mudprone interval and is overlain by at least 10 m (33 ft) of mudstones; both mudstone units host numerous smallscale sand injections (Figure 7). The sandstone unit, which is only recognized in well 15/9-A-26, has an anomalous position in the stratigraphy, within mudstones of the younger Lista Formation (Figure 7, cored interval 2605–2606 m [8546–8549 ft]). However, subsurface formation pressure data indicate that the sand is in complete pressure communication with the underlying main reservoir interval (Figure 4). Within the mudstone units, sandstone dikes and sills are common, with many injected sandstones incorporating depositional mudstone as angular or platy clasts (Figure 7).
FIGURE 7. Well 15/9-A-26. Homogeneous sandstone unit (s1) located anomalously within the stratigraphy. Overlying this sandstone is a mud-rich interval (Lista Formation) that has numerous sandstone injectites, including dikes (s2) and sills with incorporated mud clasts (s3, s4, and s5) within it. For the location of cores, see Figure 5. Bedding is near horizontal in the core. Depth shale (m MD) refers to depth below platform measures along the wellbore.
Wells 15/9-A-8 and 15/9-A-22
Several sandstone units have intervals with subvertical laminae that are defined by variations in grain size (Figure 8A) and/or variations in grain packing (Figure 8B); upward distortion of the laminae is observed. The oversteepened dish structures (Figure 8B) indicate that dewatering (liquefaction) processes predated the formation of the subvertical laminae.
FIGURE 8. (A) Core from well 15/9-A-22 illustrating subvertical laminae (white arrow) below a shale layer. The stratigraphic dip of beds is 3 –58 (black arrow), and well deviation is 458. (B) Subvertical laminae from well 15/9-A-8. Deformed dish structures (black arrow) indicate that sand fluidization occurred after water escape and liquefaction. The stratigraphic dip of beds is 3 –58, and well deviation is 408. The white bar in both parts of the figure is 5 cm (2 in.) long. Depth shale (m MD) refers to depth below platform measures along the wellbore.
Well 15/9-D-3 H
A high concentration of subangular siltstone and mudstone clasts is supported within a fine-grained sandstone matrix (Figure 9). The clasts range in size and appear spatially disorganized; however, they may sometimes show a jigsaw configuration with filamentous projections present along some clast boundaries (Figure 9A). Frequent sand-filled fractures are recorded within the siltstone and mudstone clasts (Figure 9B). This siltstone and mudstone clast breccia overlies homogeneous sandstone with a diffuse boundary between them. Because of the limited lateral view offered by the core section, the upper boundary of the breccia is difficult to define. The boundary may be a mudstone unit (Figure 9A, 2502.6 m [8210.6 ft] core depth), in which case the boundary is sharp and discordant to bedding, or it may be transitional into overlying homogeneous sandstone (Figure 9A, 2501 –2502.5 m [8205.3 –8210.3] core depth); it cannot be determined if the mudstone unit is in situ. If the mudstone unit is simply a large clast that happens to be oriented approximately bedding parallel, a case can be made for the complete disintegration of a formerly thicker depositional mudstone.
FIGURE 9. Well 15/9-D-3 H. (A) Angular, disorganized mud clasts supported by a sandstone matrix, forming a mudstone clast breccia. The lower boundary is transitional. The upper boundary is less clearly defined but may be discordant and sharp (white arrow), or if this mudstone is a clast, the boundary appears to be transitional into the overlying homogeneous sandstone. (B) Several of the mudstone clasts are partly or completely dissected by small sand intrusions (white arrow), and larger sandstone injectites are also evident (black arrow). White bar is 5 cm (2 in.) long, and bedding is approximately at 308 in the core. Depth shale (m MD) refers to depth below platform measures along the wellbore.
DEPOSITIONAL ENVIRONMENT
The reservoir sandstones were deposited in a sand-rich submarine fan environment on the western margin and southern termination ofthe Utsira High in the eastern part of Block 15/9. They were soureed from the East Shetland Platform and transported east and southeastwards, controlled by the basin floor topography in the Southern Viking Graben, and deposited as sandlobes, possibly scoured and channelised, at the gentie dipping slope of the Uts ira High margin. The sandstones, which can be up to 140m in total thickness, display a blocky weil log pattern ofthick sandstones (up to several tens ofmetres), separated by -I m thin mudstones giving an overall net/gross ratio of 0.88. In a total of 740m of cores 7 lithofacies and 20 subfacies have been recognised, of which the medium to fine-grained sandstones, massive and liquefied deposited from high-density turbidity currents are the most prevailing. The intervening mudstones represent hemipelagic deposits.
STRATIGRAPHIC ORGANIZATION
The sandstones are attributed to the Ty Formation and are older than the sandstones on the Sleipner Terrace further west in Block 15/9, which belong to the Heimdal Formation. Deposition took place during a 3 Ma time period. In a sequence stratigraphic perspective the Palaeocene sandstones ofthe submarine fan complex are interpreted to consist oftwo stacked lowstand fan cycies of intermediate order (3rd order?) bounded by sequence boundaries, which reflect significant changes (drops) in relative sea-level at the Shetland Platform. At a smaller scale these two cycles can be further subdivided into single or multiple, short-term order (4th order?), lobe-complex depositional units. A total of nine such chronostratigraphically constrained units have been identified, each consisting of one thick sandstone (or several sandstones separated by thin mudstones) overlain by a thin mudstone that can be recognized and correlated over the whole Sleipner 0st Field area. Sand-deposition took place during lowstands and mudstones were deposited as mud blankets during periods of lobe-abandonment related to lobe switching and/or rise in relative sea-level.
Since the sandstone deposits are very uniform and often barren ofmicrofossils, the biostratigraphic data from the mudstones has been invaluable as a support in the correlation process. Of a total of 33 fossil events identified, 12 are regarded as more reliable for correlation and have been part ofthe identification ofthe depositional units.
Units TI-T5 ofthe first fan are associated with green calcareous hemipelagic mudstones (maris), while Units T6(A&B)-T8 are associated with greyish black to green non-calcareous hemipelagic mudstones indicating shallower water depths, more stagnant sea-bed conditions and a stronger influence by a terrestrial souree as indicated by the fossil material.
The various units represent variable complexity and times of deposition. Compensation sedimentation of lobes with a general preferred NW -SE orientation and an overall basinward to shelfward stepping pattern of units within the two lowstand fans is observed. The identified units constitute the reservoir zones ofthe geological model which generally have sheetlike to elongated geometries.
Data source: Sleipner 0st Field a sand-rich palaeocene gas-condensate reservoir offshore Norway-sedimentology, stratigraphy, heterogeneity and paleocontact influence on reservoir properties, flow and production. Susan K. Strommen, Christian Halvorsen, Valerie Langlais, Gitte V. Laursen. 1998
Sand-injection Structures in Deep-water Sandstones from the Ty Formation (Paleocene), Sleipner Øst Field, Norwegian North Sea. Nicholas Satur, Andrew Hurst. 2007
Следующее Месторождение: HOD