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Remobilization and Injection in Deepwater Depositional Systems

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Remobilization and Injection in Deepwater Depositional Systems 5HPREOL]DWLRQ DQG ,QMHFWLRQ LQ 'HHSZDWHU 'HSRVLWLRQDO 6\VWHPV ,PSOLFDWLRQV IRU 5HVHUYRLU $UFKLWHFWXUH DQG 3UHGLFWLRQ Lidia Lonergan Nick Lee Howard D. Johnson T.H. Huxley School of Environment Earth Sciences and Engineering Imperial College of Science Technology and Medicine Prince Consort London SW7 2BP UK [email protected] Joe A. Cartwright Department of Earth Sciences Cardiff University Cardiff CF1 3YE Wales Richard J.H. Jolly Golder Associates (UK) Limited First Floor, Clyde House Reform Road, Maidenhead Berkshire SL6 8BY UK $EVWUDFW ‘parent’ sand body must be sealed such that an overpres- sure with a steep hydraulic gradient can be generated. The Several productive Paleogene deepwater sandstone res- seal on the overpressured sand body must then be ervoirs in the North Sea show evidence of having breached for the sand to fluidize and inject. The stress undergone post-depositional remobilization and clastic state within the basin, burial depth, fluid pressure and the injection, which can result in major disruption of the pri- nature of the sedimentary host rock all contribute to the mary reservoir distribution (e.g., Alba, Forth/Harding, final style, geometry and scale of intrusion. At shallow Balder, and Gryphon fields). Case studies of deepwater depths, within a few meters of the surface, small irregular sandstones from UK Quadrants 9, 15, 16 and 21 are pre- intrusions are generated, more commonly forming sills, sented to illustrate the wide spectrum of remobilization whereas at greater depth larger and more continuous dikes features, which range from centimeters (e.g., core-scale) to and sills form clastic intrusion networks. Field examples hundreds of meters (e.g., seismic-scale). Most common from the Ordovician in Ireland, and Panoche Hills in Cali- are clastic injection structures such as dikes and sills. Sills fornia are used to illustrate the control of burial depth/ of massive sand, over 20 m thick, have been identified. stress on intrusion scale. Intrusions associated with the propagation of syn- to early Earthquake induced liquefaction, tectonics stresses and post-depositional, dewatering-related polygonal fault sys- build-up of excess in-situ pore pressure are the most com- tems in adjacent deepwater mudrocks are also common. monly cited explanations for the occurrence of clastic The scale of the clastic intrusion and remobilization has intrusions. However, our work suggests that the large- significant impact on reservoir architecture and production scale, ‘catastrophic’ sandstone intrusions within the North performance, including changes in (a) original deposi- Sea Paleogene, which remobilized hundreds of cubic tional geometries; (b) reservoir properties; meters of sediment, probably require the presence of fluids (c) connectivity, (d) top reservoir surface structure, (e) res- migrating from deeper within the basin (e.g., gas charge) ervoir volumetrics, and (f) recovery/performance to drive the injection. Deepwater sand bodies within the predictions. North Sea that appear most susceptible to remobilization There are several prerequisites for sandstone intrusions occur in mud-dominated successions and include (1) nar- to form: the source sediment must be uncemented, and the row, elongate channel or gully-filled sands (i.e., non- GCSSEPM Foundation 20th Annual Research Conference 515 Deep-Water Reservoirs of the World, December 3–6, 2000 Remoblization and Injection in Deepwater Depositional Systems: Implications for Reservoir Architecture and Prediction leveed channel systems), and (2) isolated sand-rich spectrum of processes ranging from the redistribution of mounds (e.g., ‘ponded’ sand bodies and terminal fan sand at its original depositional location (e.g., the sand lobes). Sand bodies located above rift-related basin-form- mounds described by Brooke et al., 1995) through injec- ing faults, which periodically appear to have acted as tion, to extrusion of injected sand out onto the sea bed vertical fluid escape pathways, were especially susceptible (such as pock-mark craters described by Cole et al., 2000). to remobilization. Sand remobilization may influence res- As a direct result of remobilization and injection, many ervoir distribution in other mud-dominated, deepwater North Sea Paleogene reservoirs have more complex geom- depositional systems. etries than would have been the case if only primary depo- sitional processes had been responsible for their ,QWURGXFWLRQ formation. This, coupled with their subtle expression on seismic data and with their lack of primary depositional characteristics (e.g., as seen in core), make the predictabil- Large-scale sandstone remobilization and injection ity of these deposits difficult, and they remain challenging 3 (km -scale) are important components of the deepwater prospects for exploration and appraisal. For example, sand play within the latest Paleocene and early Eocene of the remobilization processes can result in significant changes North Sea Basin. These soft-sediment deformation pro- to the reservoir geology, as summarized below (Fig. 1): cesses have directly affected at least ten significant hydrocarbon accumulations within an area of ~500,000 1. Reservoir architecture (Fig. 1A): Typical features km2 in the Central and Northern North Sea (e.g., Forth/ include steepening of original depositional geometries Harding, Alba, Balder, Gryphon fields: Alexander et al., (e.g., Balder Field, Jenssen et al., 1993; Rye-Larsen, 1993; Jenssen et al., 1993; Newman et al., 1993; Newton 1994); development of pod-like sandbodies (e.g., and Flanagan, 1993; Timbrell, 1993; Dixon et al., 1995; Balder and Alba Fields); vertical intrusion of clastic Lonergan and Cartwright, 1999; Lawrence et al., 1999; dikes and sills above the reservoir (e.g., Forth/Harding, MacLeod et al. 1999). Nevertheless, the traditional inter- Frigg, Gryphon and Alba Fields; Newman et al. 1993; pretation of North Sea Tertiary deepwater sandstone Newton and Flanagan, 1993; Dixon et al., 1995), and bodies is based on the assumption that reservoir distribu- sand intrusion up faults along the reservoir margins tion and heterogeneity reflect a primary depositional (Alba Field, Lonergan and Cartwright, 1999; MacLeod origin. Here is reviewed the mounting body of evidence, et al., 1999). which demonstrates that deepwater sandstone geometries 2. Reservoir properties (Fig. 1B): Homogenization of can be significantly modified by post-depositional remobi- sand texture and reservoir properties (e.g., by clay elu- lization. An awareness of the processes and products of triation), and obliteration of original sedimentary sandstone remobilization should be incorporated within structures leading to a massive sandstone facies. These the current spectrum of deepwater depositional models facies are often indistinguishable from originally- (e.g., Reading and Richards, 1994), particularly the poten- deposited massive sandstones, which would be tradi- tial for modifying primary sand body geometries and, in tionally interpreted as the deposits of either sandy extreme cases, completely controlling reservoir grainflows, debris flows or high-density turbidity cur- distribution. rents (e.g., Lowe, 1982; Pickering et al., 1995; Shan- We review several seismic-case studies from the North mugam et al., 1995; Shanmugam, 2000). Sea hydrocarbon province, previously unpublished core 3. Sand body connectivity (Fig. 1C): Clastic intrusions data and selected outcrop analogues to illustrate the range can alter the transmissivity, typically by allowing con- and scale of clastic remobilization and injection features nectivity between previously isolated reservoir units. within deepwater depositional environments. The controls However, vertical or steeply dipping dikes, which on clastic intrusion formation from a theoretical perspec- enable this improved connectivity, will be difficult to tive are also reviewed, and we consider why sandstone image on seismic data. Hence, connectivity between intrusion and remobilization are so widespread within the apparently separate sand bodies may not be evident upper Paleogene sedimentary rocks of the North Sea. initially, and usually requires dynamic reservoir data (e.g., pressures and/or well test information) to confirm Remobilization and its effect on reservoir geology transmissivity. 4. Top reservoir structure/depth surface (Fig. 1D) (e.g., Remobilization can be defined as soft-sediment defor- Alba Field, MacLeod et al., 1999): Modifications to mation in the sub-surface during early burial, which the top reservoir surface typically involve a much mainly occurs by the forcible injection of sandy sediments higher degree of small-scale, ‘structural’ variability. into a fine-grained host-rock. However, we consider the For example, injection at the top of a parent sand body term remobilization as being broad enough to cover a results in a potentially highly irregular contact between 516 , 517 e.g. , no e.g. yyy yyyyy yyy yyy yyyyy yyy yyyyy yyyyy Sill Dike yy yyy yyyyy yyyyy Massive blocky Massive sandstone yyy yy log gamma-ray yyyyy yyyyy yy yyy yyyyy yyyyy Ye s yy yyy yyyyy yyyyy yy yyy yyyyy No yyyyy yy yyy yyyyy yyyyy yyy yyy OWC No internal breaks clay yyyyy OWC sills superficially resemble thin-bedded turbidites ( ‘ratty’ sands on gamma ray logs). However, sands However, logs). ray gamma on sands ‘ratty’ are much morewithin in erratic injection complexes their and lateral vertical do they distribution, not dis- ( successions facies vertical play predictable thickening thinning or trends) upward and
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