A revised Chalk lithostratigraphic nomenclature A REVISED CHALK LITHOSTRATIGRAPHIC NOMENCLATURE Astri Fritsen & Fridtjof Riis, Norwegian Petroleum Directorate The petroleum activity in the Noth Sea over the years revealed a need to update and revise the stratigraphic framework for the chalk units. The presence of "time gaps" and redeposition in the chalk reservoirs especially challenged the the established formal lithostratigraphic nomenclature. Biostratigraphic analysis combined with chalk lithofacies, well log correlation and seismic studies were used to develop a revised complete lithostratigraphic nomenclature for the Upper Cretaceous and Paleocene chalks in the Central Graben. The most important change is the subdivision of the original formal Hod Formation into three new formations; Narve, Thud and Magne. This subdivision is based on the major stratigraphic breaks that were observed in the Hod Formation. These breaks represent time gaps of variable lengths in the biostratigraphic data, and are often seen as hardgrounds in cores, and especially in wells located on structural highs. Also in the seismic data, the Hod Formation was seen to embrace two significant sequence boundaries that could be tracked over large distances. On basis of international stratigraphic rules (Hedberg 1976), such major sequence boundaries are not recommended within a formation. Consequently, the split of the Hod Formation into three new formations was performed. To avoid confusion with the existing informal lithological subdivision of the Hod into lower, middle and upper Hod, unique names for the new units were suggested, following the tradition with names from the Norse mythology. The boundary definition of the Tor Formation has been revised, and the Vidar Formation overlying the Ekofisk Formation has been better defined. data. The resulting joint biostratigraphic INTRODUCTION and lithostratigraphic framework for the Extensive reservoir modelling and drilling chalks in the Central Graben is presented activity in the chalk areas of the North Sea in this publication, and a revision of the the last twenty years has resulted in many formal Hod Formation into three new different ways of subdividing the chalk formations; Narve, Thud and Magne is reservoirs. The oil companies developed proposed, see Figure 1. The detailed their own local stratigraphic frameworks or documentation of the basis for the nomenclatures in order to fit the well data stratigraphy, including biotrat analyses and from their license areas. This practice composite well log panels, can be found in complicated communication between oil the report (Fritsen et al 1999), available on companies and the regional understanding compact disc from the Norwegian of the chalk facies and created a need to Petroleum Directorate. update and revise the formal stratigraphic framework for the chalk units. The new stratigraphic nomenclature will improve the communication between the In the Joint Chalk Research (JCR) phase oil companies in both Norwegian and V, 1997 – 2000, the project “A Joint Chalk Danish areas, and ensure a standard basis Stratigraphic Framework” was initiated in for regional mapping and correlations order to establish a common stratigraphic between the various chalk fields and nomenclature for the Upper Cretaceous prospects. Since the presence of "time and Paleocene chalks in the Central gaps" and redeposition in the chalk Graben and Norwegian-Danish Basin. The reservoirs is especially acknowledged in work was performed by a joint work group the proposed new lithostratigraphic including geologists from the companies nomenclature, improved understanding of and agencies participating in JCR Phase V. deposition, erosion and redeposition of Biostratigraphic analyses comprised the chalk, as well as better control of seismic majority of the project work, and the interpretation on and between fields will results were interpreted together with well result. logs, chalk depositional facies and seismic - 1 - A revised Chalk lithostratigraphic nomenclature TIME Danish JCR JCR North Sea UK SCALE informal NANNO. proposed formal formal Gradstein AGE Chalk units ZONES FORMATIONS FORMATIONS FORMATIONS et al ., Isachsen and Lieberkind et.al., this study this study Lott and Knox, 1994 1995 Tonstad, 1989 1982 60.00 59.00 LISTA / VIDAR LISTA / VIDAR THANETIAN NNTp6 - 9 NORTH SEA MARL L. VÅLE PAL. VÅLE MAUREEN 61.00 NNTp5 NNTp4 NNTp3 EKOFISK DANIAN EKOFISK CHALK-6 UNIT NNTp2 EKOFISK CENE EARLY 65.00 PALEO- NNTp1 UC20 CHALK-5 UNIT TOR MAASTRICHT. UC19 TOR ROWE 70.00 UC18 CHALK-4 UNIT UC17 71.30 UC16 UC15 MAGNE CAMPANIAN CHALK-3 UNIT 80.00 JUKES UC14 HOD 83.50 UC13 THUD SANTONIAN UC12 85.80 UC11 LATE CRETACEOUS CONIACIAN UC10 LAMPLUGH 89.00 CHALK-2 UNIT 90.00 UC9 NARVE TURONIAN UC8 UC7 UC6 HERRING 93.50 UC4 & 5 BLODØKS BLODØKS UC3 CENOMANIAN UC2 HIDRA HIDRA CHALK-1 UNIT HIDRA 98.90 UC1 Figure 1: A revised chalk lithostratigraphic nomenclature quantitative or semi-quantitative METHOD nannofossil- and micropaleontological A number of 35 wells from Norwegian, analyses were used in the study. Regional Danish and UK areas were chosen for the seismic was interpreted to resolve study. Biostratigraphic data, well stratigraphic surfaces between fields and interpretations and core descriptions from key wells, and chronostratigraphic these wells were made available from the diagrams were made to identify companies. Cores and wireline logs were stratigraphic time gaps and sequence studied by the work group, and intervals boundaries in the study wells. representing possible stratigraphic sequence boundaries, time gaps or Selection of the study wells was based on redeposition were defined. In total, 1250 various criteria, including the well´s meters of core were described based on the position regarding basin and structural previous developed JCR description highs, and available core and system. With more than 700 new core biostratigraphic data. Existing stratigraphic samples collected during core descriptions, type wells, such as 1/3-1 was also a total of more than 4000 biostratigraphic included. Wells that had core coverage - 2 - A revised Chalk lithostratigraphic nomenclature across a formation boundary were Figure 2 shows the location of the study considered especially important, as were wells and the chalk fields in the North Sea wells that appeared to have been drilled Central Graben. across major stratigraphic time breaks. 2 3 4 5 7/11 57 00 57 00 2/2-2 1/3-1 1/3-8 2/2-3 56 45 JUDY 56 45 2/5-7 30/7A-2 2/5-1 TOR 2/4-B-19T2 2/5-9 2/4-A-8 EKOFISK 56 30 56 30 TOMME- 2/7-4 2/7-30 LITEN EDDA 1/9-1 2/7-B-11 ELDFISK 2/7-8 LULU-1 VALHALL 2/8-A-1 MONA-1 56 15 56 15 2/7-2 HOD T-1 2/11-A-2 SVEND 31/26A-10 BARON-2 SYD-ARNE 56 00 56 00 VALDEMAR ADDA BO-1 ADDA-2 ROAR-2 55 45 55 45 ROAR E - 4X TYRA DAN 55 30 55 30 MFB-7 M-9X 2 3 4 5 0 20 40 Km RDN9909002/6 Figure 2: Location of the study wells and chalk fields tectonics and inversion tectonics added to STRUCTURAL AND TECTONIC the structural complexity. Inversion highs, FRAMEWORK AND SEISMIC formed by uplift and reverse movement INTERPRETATION along pre-existing faults, were active at different occasions during and after chalk The Central Graben was tectonically active deposition, and the detailed timing of their during deposition of the chalk sequence. activity may differ. They often represent a Graben subsidence, inversion structures change in tectonic setting from a pre- and salt tectonics created a complex and Campanian trough to a Maastrichtian high. changing pattern of basins and highs in the Late Cretaceous and Danian. Figure 3 shows the Central Graben and its main structural elements based on During chalk deposition, the Central Anderson (1995), Britze et al. (1995) and Graben subsided along its major boundary data from the Chalk Exploration Project faults towards the stable platforms. There (CEP), which was made available for JCR were several episodes when the relatively by Phillips Petroleum Company. uplifted platforms were eroded, and acted as source areas for redeposited chalk. Salt - 3 - A revised Chalk lithostratigraphic nomenclature LATE CRETACEOUS STRUCTURAL ELEMENTS IN THE CENTRAL GRABEN 2 3 4 5 7/11 Late Cretaceous 57 00 57 00 boundary faults 2/2-2 Late Cretaceous 1/3-1 active inversion faults 1/3-8 Inverted highs 2/2-3 56 45 56 45 Salt induced 2/5-7 structures 30/7A-2 2/5-1 Stable platform thin chalks 2/4-B-19T2 2/5-9 Basin areas 2/4-A-8 56 30 56 30 2/7-4 M Deep basin 2/7-30 A N depocentre 2/7-B-11 D 1/9-1 A 2/7-8 L H Main pre-Cretaceous IG LULU-1 H fault structures L M I 2/8-A-1 O MONA-1 N N 56 15 2/7-2 D A 56 15 E R S IDGE N ES R T-1 2/11-A-2 ID GE 31/26A-10 BARON-2A R N 56 00 56 00 E R I ID NG G E H E IG H B O - J E N BO-1 ADDA-2 S R ROAR-2 55 45 I 55 45 D G E - 4X E 55 30 55 30 MFB-7 M-9X Figure 2-1 2 3 4 5 0 20 40 Km RDN9909002/6 Figure 3: Central Graben main structural elements support correlation based on logs and A major depocenter for the Cretaceous biostratigraphy. Seven sequence chalk located in the northwestern part of boundaries were interpreted to determine the Central Graben is penetrated by wells the important seismo-stratigraphic events 1/3-1 and 1/3-8. These wells have a fairly in the area, these were Blodøks, Intra complete chalk section, and they are Middle Hod, Top Middle Hod, equivalent important reference wells.