Slide 1

LITHOSTRATIGRAPHY AND THE CRETACEOUS OF THE NORTH SEA A Brief Outline of Concepts It is not Easy! Stephen Crittenden Independent Geological Consultant Slide 2 TALK STRUCTURE oDefine Stratigraphy oOnshore and Offshore oDefine lithostratigraphy oCretaceous Subsurface lithostratigraphy schemes oCriteria and Method oForward and Onwards Slide 3 FIRST, WHAT IS STRATIGRAPHY? oStratum = Latin oGraphia = Greek As trained geologists we are all familiar with stratigraphy. Defined as the description of all rock bodies forming the earths crust and their organisation into distinctive, useful, mappable units based on their inherent properties or attributes in order to establish their distribution and relationship in space and their succession in time, and to interpret geological history. Slide 4 Stratigraphy Conventional Purist Stratigraphy Concept Stratigraphy comprises: Lithostratigraphy Biostratigraphy Chronostratigraphy Sequence Stratigraphy Seismic Sequence Stratigraphy Magnetostratigraphy Other stratigraphies based on other properties of rock bodies. For example Flow Units, Chemical Composition, Heavy Minerals Slide 5 STRATIGRAPHICAL UNITS Table of units Slide 6 Stratigraphy Models As working petroleum geologists we deal extensively with sub- surface data rather than outcrop data. For the North Sea explorer we deal exclusively with offshore material We must still retain a classical academic approach to stratigraphy first principles, to underpin our Pragmatic, Working Stratigraphy. We have to work with a pragmatic philosophy. Often the stratigraphies erected by the industrial petroleum geologist are neither strictly defined nor procedurely correct. They are an immediate adequate means to an end but inevitably will cause problems in the future. Stratigraphy is an important part of the geologists tool-box used by explorationists to find hydrocarbons. Do it Right. But, not all geologists are stratigraphers! Slide 7 OFFSHORE STRATIGRAPHY The key to the offshore is the onshore Slide 8 OFFSHORE STRATIGRAPHY Stratigraphical schemes for the offshore are constructed / erected with reference to the vast database of onshore information. This can be illustrated by reference to the lithostratigraphy of the Lower Cretaceous Albian Stage. Slide 9 Lowermost part of the Rodby Formation & Upper part of Sola Formation equivalent onshore UK Slide 10 The Red Chalk Formation and Rdby Formation onshore equivalent = the Hunstanton Formation Slide 11 The Albian Lithostratigraphy Offshore Slide 12 WHAT IS LITHOSTRATIGRAPHY? The part of stratigraphy that describes and names rocks based on lithology and stratigraphical relationships and the use of that data to organise rock bodies into lithostratigraphical units. Lithostratigraphy is only part of the overall picture seen by the petroleum geologist. Lithostratigraphy is an important building block for model generation in the search for oil and gas. The aim of the petroleum geologist is the generation of an overall, all-encompassing stratigraphy model which aids in the search for oil and gas. Lithostratigraphy is a part of that model. Slide 13 Cause and Affect Philosophy Common Causes in the environment of deposition affect Lithology Faunas & Floras Which if all other factors are equal, in turn control LWD response data Fossil Assemblage data And are interpreted to derive Drilling data incl. ROP, Torque, Gas LithostratigraphyBiostratigraphy Which all together produce INTEGRATED EVENT STRATIGRAPHY Slide 14 Lithostratigraphy: a part of Stratigraphy - a tool in the geologists tool-box Slide 15 THE PRESENT IS THE KEY TO THE PAST An Ideal Solution An instant later the time travelling stratigrapher, with his thermometer, is obliterated leaving the warm blooded / cold blooded dinosaur debate unresolved Slide 16 Formal Lithostratigraphy Classification An established Conventional Unit heierarchy. Units are recognised by observable physical features. Group comprises two or more contiguous or associated formations. Associated groups may be part of a Supergroup. Formation the primary unit of lithostratigraphy recognised solely on lithology. It has to be mappable. Member a named lithological subdivision of a formation that may extend into other formations. Bed a named distinctive layer in a member or formation. A key bed or a marker bed. Unit Boundaries do not define time lines. Fossil content may be a diagnostic lithological component. Slide 17 ESTABLISHING LITHOSTRATIGRAPHICAL UNITS Procedure Type localities / Stratotypes clear and precise. With auxillary reference sections. Boundaries positions of lithological change. In the subsurface define the boundary at the top occurrence of the particular rock type. Slide 18 Cretaceous Subsurface Lithostratigraphy Schemes A Progression from the simple to the complex. Use both onshore surface and subsurface data with offshore subsurface data. As a basin is explored the schemes erected become more and more detailed. Schemes developed initially are parochial as each Oil Company and each country involved in the basin erect their own secretive models. Later Co-operation results in better understanding, data sharing and in better models. Slide 19 Lower Cretaceous Lithostratigraphy Models Compared North Sea Slide 20 Stratigraphical Synthesis Lower Cretaceous Slide 21 The Albian Stage Rodby Formation onshore & Offshore UK Slide 22 Upper Cretaceous Lithostratigraphy Models Compared North Sea Slide 23 Subsurface lithostratigraphy units and schemes In practise identified and / or defined by the petroleum geologist using both: Lithology and LWD / Wireline log shapes Slide 24 Pragmatic Lithostratigraphy The Petroleum Geologist utilises all data at the wellsite for identification of lithostratigraphic units; formal and informal. Slide 25 Example Conventional Lithostratigraphy Formations Rogaland Group Hordaland Group Ekofisk Formation is part of the Shetland Group 1. Palaeocene section over the crest of the structure is incomplete from the base upward. 2. Formations pinch out around and onto the flanks and are absent on the crest. This accounts for the lack of the Vaale and Ekofisk formations over the crest of the structure. 3. This also accounts for the patchy distribution of the Cenodiscus Claystone Member over the structure. Pinch-out (onlap / offlap) of the structure by the lower part of the Palaeocene section has a major impact on casing pick. The amount of section to be drilled through the Lista Formation is going to be variable. Paleocene section on flanks of structure is more complete and usually includes Vaale and Ekofisk formations. Informal Member status, usually at wellsite can only be reliably identified by palaeo Slide 26 LWD Log Lithostratigraphy Idealised Events Crestal location Clyst: lt gy gy, lt blu grn gy, frm, blky sub fiss, silty, sli calc, tr mica, glc. Stringers of Dol, reddish brn, brn gy, off wh, hd, xln. Stringers of Chalky Lst, wh lt brn, frm hd, brit, sucrosic. Associated with increase in gas values. Top Balder Formation 2613mMD, -2367m False Balder Formation peaks = reworked horizon (s). GR response background trend change; trend slowly increases in value downhole. Top Sele Formation 2625mMD, -2378m Top Lista Formation 2634mMD, -2386m Top Tor Formation 2653mMD, -2403m Tuff tuffaceous clyst: lt med gy, speckled wh gy, blky, sft friable, sli calc, qtz incl. Clst: varicoloured grn, lt grn. Clst: dk - lt brn earthy brn, lt-dk grn interbeds, sft, slty, pyr. Clst: varicoloured, lt grn, dk bluish grn, dk grn, gy, lt gy, sft- frm, blky, smooth, waxy appearance, pyr, glc/chl, siderite. Lst intbeds, gy- wh. Clst, red brn, chocolate brn, = Lower Red Marker. GR response trend is rather flat. Upper Red Claystone Marker. Clyst more varicoloured bluish, prplsh with depth. Reworked tuffs. Gas peak from Balder = higher resistivity. GR peak characteristic of some crestal wells. Fork peak. GR values higher than Balder. GR Bow: peak, cutback, peak. Slide 27 Subsurface Lithostratigraphy Models Good cuttings quality for Lithology description. Accurate lag time. Caught on depth. Hole in gauge and well cleaned. No caving. Close interval: 3m intervals. Spot cuttings samples when necessary. Good Core Data is an ideal. Good Drilling Data ROP, WOB, Torque. GR & Resistivity response from LWD. Good data quality. Slide 28 Lithological Description from cuttings Undiff E. Eocene interval Upper Red Claystone Balder Formation Tuffs Sele Formation Lista Formation Lower Red Claystone Chalk Slide 29 Pragmatic approach for correct identification of the lithological unit. Use all available data. It is best to be prepared! Slide 30 Cretaceous Lithostratigraphy Monotonous Stratigraphy ? after all, its all white and Chalk is Chalk isnt it? anyway the Lower Cretaceous is all claystone Slide 31 Points of View It is useful to step back and gain a broader perspective Its the Herring Formation Actually, its the Chalk Group (and its a mammoth not a herring!) Its the Plenus Marl G0 Bed Slide 32 Chalk Lithostratigraphy Variety of schemes for various regions of the North Sea Chalk interfingers with the Shetland claystones. The previously illustrated slide (22) shows the complexity and detail of the lithostratigraphy. Each lithostratigraphical unit has to be illustrated by a type well section. For Norlex: we have to use the existing defined and published type well sections. Norlex can illustrate and designate reference wells. Chalk is not monotonous: there are subtle and mappable differences. Slide 33 Chalk Lagerdorf Saturn Quarry, Germany The white stuff we drill through. Some layers and fractures / faults visible. Slide 34 Faulted white stuff Lagerdorf Saturn Quarry Slightly more interesting well defined lithological layers and clear faulting. Slide 35 Reservoir Stratigraphy Geological Model for a Chalk reservoir is complex; it is not a simple layered cream-cake. Faults and Fractures, both small scale and large scale are present but not all are possible to model from seismic. Slide 36 Field Reservoir Stratigraphy Perhaps there is more to Chalk than meets the eye! Detailed Stratigraphy flow units, lithostratigraphy and biostratigraphy Slide 37 THE KEY TO SUCCESS IS TEAM WORK Slide 38 Lithostratigraphical studies of the Cretaceous Is it a success story? Effective people Effective data and QC of data Effective acquistion and interpretation of data Effective modelling Effective software Effective communication Effective documentation Objective: A holistic understanding of all the subsurface data and how it all slots together to achieve a lithostratigraphic model. Slide 39 Forward Vision Clear vision, rationale and plan for the process of building an effective Lithostratigraphic Model. Review Model sessions. Feedback, consolidate and discuss. Re-define vision, strategy (ies) and methods. Identify any changes necessary Identify any impediment to change Identify any other resources needed: skills, tools, people. Time frame required for change Slide 40 Back up picture Slide 41