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 overlainRATE 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 struc￾tures 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 knowledge￾able 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 sand￾stones 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-southeast￾trending faults during the deposition of the Triassic. In the Piper area, the Triassic red shales are overlain