《石油地质学 Petroleum Geology》课程教学资源（阅读文献）Piper Field-U.K. Outer Moray Firth Basin, North Sea.pdf_P6-P9

Viking graben. Instead the virgin Outer moray Firth EFFICIENT RESERVOIR MANAGEMEN basin blocks were nominated as their first choice VALNTENANCE SHUTDOWN As explained above, following the successful 250 application for the acreage the initial two wells drilled in this virgin basin, 15/11-1 and 14/19-1, had Tertiary sandstones as their primary objective with the integration of the results of these wells, the primary objective of the Piper discovery well, 15 17-1A, were Upper Jurassic sandstones PLATFORM REPA RS Now 6770798a118384858678 With todays exploration methods, Piper field would be easily mapped and discovered. Since the discovery of the Piper field, the industry has become WATER INJECT very much more knowledgeable about extension-type Figure 6. Graph showing Piper fields performance basins and the stratigraphy, origin, migration, and story,1976-1988Production averaged over 250.000 entrapment of oil and gas within them It would be easy to map Piper-type structures and BOPD for the first three years but had declined to approximately 120,000 BOPD by mid-1988, 62% available, to recognize that the reservoir sandstone watercut, at the time of the disaster. Water production and caprocks are present. It still is not possible to exceeded oil production in 1987. tell which undrilled structures contain oil or gas Today, Occidental's first choice structure in block ill might be drilled first, as it was in 1972, and be found dry Although not definitely proven by Petronella, Highlander, Scapa, Rob Roy, Ivanhoe and the 15/11 structure is dry because it remained too Chanter fields. Several additional discoveries have high during the Cretaceous. Consequently, the Piper yet to be declared commercial Figure 1B) sandstones we slapped by the maastrichtian chalk, which is not a caprock in this area. Indeed the chalk came to within a few feet of onlappir the sandstone at Piper, in which case the Pipe DISCOVERY METHOD structure would also have been dry Prior to the application for acreage that contains STRUCTURE the Piper field, the Occidental exploration team had 30. 400 km) of seismic data covering large areas of Tectonic History the U. K northern North Sea. The prospectivity of The North Sea and northwest Europe were part this region was considered to be good, based on the of the landmass until the late Permian(Ramsbottom knowledge of existing source rocks and reservoir 1978). At this time a shallow epicontinental gulf sandstones cropping out onshore and on the fact that covered most of the north sea basin area and resulted two large fields had already been discovered (Ekofisk in widespread evaporite deposition In the southern and Forties). A Mesozoic discovery that turned out North Sea, thick salt deposits of the halibut Bank to be the brent field was also rumored (Figure 1A). Formation(Zechstein) were laid down. In the pipe A large number of potentially prospective struc- area, 100 ft (30 m) of interbedded dolomite and tures were mapped within the U. K. 4th Round anhydrite containing an acreage. Seismic data quality was good, and it was Permian palynomorphs were overlain by 300 ft( 91 a reasonable assumption that nearly every operator m)of massive anhydrite containing a few dolomite he United Kingdom had comparable structural beds up to 20 ft(6 m)thick(figure 2 interpretations. However, some of the major oil The halibut Bank Formation overlies a thick companies had the advantage of having already sequence of Lower to Middle Carboniferous sand drilled wells through the Mesozoic sandstones and stones and shales It in turn is overlain by extensive had good data on reservoir quality, source rock, and sections of nonmarine shale and claystone of Triassic caprock. a key point in the Occidental Group' s age. Drilling in the Claymore field has confirmed cquisition strategy was therefore to avoid head-on structural movement on northwest-southeast competition with other larger and more knowledge. trending faults during the deposition of the triassic able operators for the structures in the northern In the piper area, the triassic red shales are overlain

RATE REDUCTION FOR MORE EFFICIENT RESERVOIR MANAGEMENT MAINTENkNCE SHUTDOWN PLATFORM REP 76 ( 77 1 78 1 79 1 80 1 81 1 82 1 83 1 84 1 85 1 86 1 87 1 86 YEAR - OIL PROD. --- WATER PROD -- WATER INJECT Figure 6. Graph showing Piper field's performance history, 1976-1 988. Production averaged over 250,000 BOPD for the first three years but had declined to approximately 120,000 BOPD by mid-1 988, 62% watercut, at the time of the disaster. Water production exceeded oil production in 1987. Petronella, Highlander, Scapa, Rob Roy, Ivanhoe, and Chanter fields. Several additional discoveries have yet to be declared commercial (Figure 1B). DISCOVERY METHOD Then Prior to the application for acreage that contains the Piper field, the Occidental exploration team had acquired and interpreted over 19,000 line miles (30,400 km) of seismic data covering large areas of the U.K. northern North Sea. The prospectivity of this region was considered to be good, based on the knowledge of existing source rocks and reservoir sandstones cropping out onshore and on the fact that two large fields had already been discovered (Ekofisk and Forties). A Mesozoic discovery that turned out to be the Brent field was also rumored (Figure 1A). A large number of potentially prospective structures were mapped within the U.K. 4th Round acreage. Seismic data quality was good, and it was a reasonable assumption that nearly every operator in the United Kingdom had comparable structural interpretations. However, some of the major oil companies had the advantage of having already drilled wells through the Mesozoic sandstones and had good data on reservoir quality, source rock, and caprock. A key point in the Occidental Group's acquisition strategy was therefore to avoid head-on competition with other larger and more knowledgeable operators for the structures in the Northern Viking graben. Instead, the virgin Outer Moray Firth basin blocks were nominated as their first choice. As explained above, following the successful application for the acreage, the initial two wells drilled in this virgin basin, 15/11-1 and 14/19-1, had Tertiary sandstones as their primary objective. With the integration of the results of these wells, the primary objective of the Piper discovery well, 15/ 17-lA, were Upper Jurassic sandstones. Now With today's exploration methods, Piper field would be easily mapped and discovered. Since the discovery of the Piper field, the industry has become very much more knowledgeable about extension-type basins and the stratigraphy, origin, migration, and entrapment of oil and gas within them. It would be easy to map Piper-type structures and, perhaps, with the quality of the seismic data available, to recognize that the reservoir sandstone and caprocks are present. It still is not possible to tell which undrilled structures contain oil or gas. Today, Occidental's first choice structure in block 15/11 still might be drilled first, as it was in 1972, and be found dry. Although not definitely proven by the three wells on this block, it is now thought that the 15/11 structure is dry because it remained too high during the Cretaceous. Consequently, the Piper sandstones were onlapped by the Maastrichtian chalk, which is not a caprock in this area. Indeed, the chalk came to within a few feet of onlapping the sandstone at Piper, in which case the Piper structure would also have been dry. STRUCTURE Tectonic History The North Sea and northwest Europe were part of the landmass until the late Permian (Ramsbottom, 1978). At this time, a shallow epicontinental gulf covered most of the North Sea basin area and resulted in widespread evaporite deposition. In the southern North Sea, thick salt deposits of the Halibut Bank Formation (Zechstein) were laid down. In the Piper area, 100 ft (30 m) of interbedded dolomite and anhydrite containing an assemblage of Upper Permian palynomorphs were overlain by 300 ft (91 m) of massive anhydrite containing a few dolomite beds up to 20 ft (6 m) thick (Figure 2). The Halibut Bank Formation overlies a thick sequence of Lower to Middle Carboniferous sandstones and shales. It in turn is overlain by extensive sections of nonmarine shale and claystone of Triassic age. Drilling in the Claymore field has confirmed structural movement on northwest-southeasttrending faults during the deposition of the Triassic. In the Piper area, the Triassic red shales are overlain

by a nonmarine sequence of siltstones, shales, Local Structure lignites, tuffs, and basalt flows with occasional thin The Piper field lies on the northern margin of the bedded freshwater limestones of Middle Jurassic age. WGG. Four significant faults are present within the In the Claymore area, and possibly the piper area of the Triassic was marke ked by a period of Piper field(Figures 3 and 8). Most of the fault erosion. Middle Jurassic sediments in the Piper area movement occurred after the deposition of the Piper basin and deposition of the widespread, lower deltaic was syndepositional. The field comprises three major plain sediments of the Sgiath Formation (Maher and tilted fault blocks, dipping gently to the northeast There was a major transgression in the late axis. The main Piper field(blocks IA and beest Harker, 1987 and slightly folded along the northeast-southwe Oxfordian over the whole of the Outer Moray Firth at about 8 to the northeast(Figures 3, 4, and 9) basin area. This is represented by the"i shale"in the Piper field(Figure 5). Following this transgres sion, the shallow marine Piper sandstones were deposited STRATIGRAPHY After the deposition of the Piper sandstones, there was major extension and extensive deepening of the The oldest sediments penetrated in Piper field are Witch Ground graben(WGG). The graben had been Lower Carboniferous Coal Measures of the Forth initiated as early as Triassic times and subsidence Formation( Figures 2 and 10). They comprise a thinly and tilting occurred during the deposition of the Piper interbedded deltaic sequence of sands, shales, and and Sgiath sandstones. Consequently, wells within coals. Upper Permian sediments, which unconform the graben have thicker Piper and Sgiath sandstone ably overlie the Carboniferous, represent evaporite sections than do wells on the northern margin of deposits of the Zechstein sea. Fluvio- lacustrine of shelf areas away from the graben margin resulted red beds of the smith Bank Formation. No Lower in erosion of Piper and Sgiath sandstones along fault Jurassic rocks are present due to nondeposition or scarps and the graben margin These sediments were erosion. The middle jurassic consists of Rattray bably redeposited as gravity flows within the Formation volcanics and Pentland Formation alluvia to marginal- marine argillaceous clastics and coals extension occurred, accompanied by a widespread bedded sands, shales, and coals of the paralic Sgiath transgression that deposited the organic-rich shales Formation were overlain by shallow marine sands of the Kimmeridge Clay Formation over the Outer and shales of the Piper Formation. These in turn Moray Firth basin area. The Piper structure were overlain by anoxic marine shales of the a result of fault block rotation and a fall in sea level brought to a close during the early cretaceous when This resulted in local erosion of Jurassic sediments sandy marls and limestones of the valhall Formation and nondeposition of Cretaceous sediments. Only a onlapped the Piper structure Diminishing tectonic thin, " condensed"sequence of Lower Cretaceous activity into the late Cretaceous was marked by deposits is present in the Piper field postrift hemipelagic deposition of marls, limestones During the Late Cretaceous, regional subsidence and chalks. Clastic sedimentation returned in the and a rise in sea level resulted in the progressive Tertiary, represented by a thick sequence of sands onlap of the Piper structure by marls and chalks of and shales Santonian, Campanian, and Maastrichtian age The Piper reservoir is composed of two formations Figure 4). By the end of the Cretaceous, tectonic within the Upper Jurassic Humber Group these are movements had largely ceased and a thick sequence the Oxfordian Sgiath Formation(harker et al., 1987) of Tertiary sands and clays was deposited throughout and the Upper Oxfordian to Kimmeridgian Piper the area. The burial history of the Piper reservoir Formation (Deegan and Skull, 1977)(Figure 10) s summarized in Figure 7. Originally the section now assigned to the Sgiath Formation was dated as Callovian(Maher, 1981), but Regional structure ised(Tur The sgiath sandstones are interbedded with shales The regional structure is an extensional graben and coals up to 10 ft(3 m)thick. Immediately over trending northwest, away from the intersection of lying the Sgiath is a silty, sandy bioturbated shale the viking and central grabens(Figures lA and IB). that grades upwards into a series of stacked shallow Extensional subsidence of the WGG basin may have marine sands. This sequence comprises the Piper aken place during the Permian and Triassic, but Formation the major phases of graben formation occurred during These sands extend over hundreds of square miles the late jurassic and early Cretaceous. The graben on the shelf in which the piper field is located and may have developed as a reactivation of Hercynian across the WGG. They shale out on the west side features of the claymore field(Boote and gustav, 1987)

by a nonmarine sequence of siltstones, shales, lignites, tuffs, and basalt flows with occasional thinbedded freshwater limestones of Middle Jurassic age. In the Claymore area, and possibly the Piper area, the end of the Triassic was marked by a period of erosion. Middle Jurassic sediments in the Piper area were also partially eroded prior to deepening of the basin and deposition of the widespread, lower deltaic plain sediments of the Sgiath Formation (Maher and Harker, 1987). There was a major transgression in the late Oxfordian over the whole of the Outer Moray Firth basin area. This is represented by the "I shale" in the Piper field (Figure 5). Following this transgression, the shallow marine Piper sandstones were deposited. After the deposition of the Piper sandstones, there was major extension and extensive deepening of the Witch Ground graben (WGG). The graben had been initiated as early as Triassic times and subsidence and tilting occurred during the deposition of the Piper and Sgiath sandstones. Consequently, wells within the graben have thicker Piper and Sgiath sandstone sections than do wells on the northern margin of the WGG. The deepening of the graben and rotation of shelf areas away from the graben margin resulted in erosion of Piper and Sgiath sandstones along fault scarps and the graben margin. These sediments were probably redeposited as gravity flows within the graben. Near the end of the Jurassic, further basin extension occurred, accompanied by a widespread transgression that deposited the organic-rich shales of the Kimmeridge Clay Formation over the Outer Moray Firth basin area. The Piper structure remained high throughout the Early Cretaceous as a result of fault block rotation and a fall in sea level. This resulted in local erosion of Jurassic sediments and nondeposition of Cretaceous sediments. Only a thin, "condensed" sequence of Lower Cretaceous deposits is present in the Piper field. During the Late Cretaceous, regional subsidence and a rise in sea level resulted in the progressive onlap of the Piper structure by mark and chalks of Santonian, Campanian, and Maastrichtian age (Figure 4). By the end of the Cretaceous, tectonic movements had largely ceased and a thick sequence of Tertiary sands and clays was deposited throughout the area. The burial history of the Piper reservoir is summarized in Figure 7. Regional Structure The regional structure is an extensional graben trending northwest, away from the intersection of the Viking and Central grabens (Figures 1A and 1B). Extensional subsidence of the WGG basin may have taken place during the Permian and Triassic, but the major phases of graben formation occurred during the Late Jurassic and Early Cretaceous. The graben may have developed as a reactivation of Hercynian features. Local Structure The Piper field lies on the northern margin of the WGG. Four significant faults are present within the Piper field (Figures 3 and 8). Most of the fault movement occurred after the deposition of the Piper sandstones, but some, notably along the "D" fault, was syndepositional. The field comprises three major tilted fault blocks, dipping gently to the northeast and slightly folded along the northeast-southwest axis. The main Piper field (blocks IA and IB) dips at about 8" to the northeast (Figures 3,4, and 9). STRATIGRAPHY The oldest sediments penetrated in Piper field are Lower Carboniferous Coal Measures of the Forth Formation (Figures 2 and 10). They comprise a thinly interbedded deltaic sequence of sands, shales, and coals. Upper Permian sediments, which unconformably overlie the Carboniferous, represent evaporite deposits of the Zechstein sea. Fluvio-lacustrine sedimentation followed with argillaceous Triassic red beds of the Smith Bank Formation. No Lower Jurassic rocks are present due to nondeposition or erosion. The Middle Jurassic consists of Rattray Formation volcanics and Pentland Formation alluvial to marginal-marine argillaceous clastics and coals. During the Late Jurassic transgression, the interbedded sands, shales, and coals of the paralic Sgiath Formation were overlain by shallow marine sands and shales of the Piper Formation. These in turn were overlain by anoxic marine shales of the Kimmeridge Clay Formation. Synrift deposition was brought to a close during the Early Cretaceous when sandy marls and limestones of the Valhall Formation onlapped the Piper structure. Diminishing tectonic activity into the Late Cretaceous was marked by postrift hemipelagic deposition of marls, limestones, and chalks. Clastic sedimentation returned in the Tertiary, represented by a thick sequence of sands and shales. The Piper reservoir is composed of two formations within the Upper Jurassic Humber Group. These are the Oxfordian Sgiath Formation (Harker et al., 1987) and the Upper Oxfordian to Kimmeridgian Piper Formation (Deegan and Skull, 1977) (Figure 10). Originally the section now assigned to the Sgiath Formation was dated as Callovian (Maher, 1981), but this has now been revised (Turner et al., 1984). The Sgiath sandstones are interbedded with shales and coals up to 10 ft (3 m) thick. Immediately overlying the Sgiath is a silty, sandy, bioturbated shale that grades upwards into a series of stacked, shallow marine sands. This sequence comprises the Piper Formation. These sands extend over hundreds of square miles on the shelf in which the Piper field is located and across the WGG. They shale out on the west side of the Claymore field (Boote and Gustav, 1987)