Triassic dryland fluvial systems occur infilling extensional and post-rift basins across northwest Europe. These systems were deposited by streams draining off the major basin margin catchments of Greenland, Fennoscandia, the Scottish Highlands and the remnant Variscan mountains. Fluvial drainage was dominantly endorheic in character, and terminated in playa, aeolian dune, sabkha or marsh settings. Whilst the ambient basin climate fluctuated through varying levels of aridity and humidity, the dispersal of sand into these basins was critically dependant on catchment run-off from the major, basin flanking regions. In the Early Triassic the Tethyan monsoon drove seasonal precipitation over the Variscan mountains which transported sediment northward into arid dune fields and playa. In addition, far-travelled fluvial systems supplied sediment from distant, wetter catchments. Through the Middle and Late Triassic the effects of the Tethyan monsoon apparently weakened, and sand-prone fluvial systems draining off the Variscan mountains were more limited in areal extent, but expanded during pluvial episodes. However, Greenland and Fennoscandia maintained sediment supply from the north, with perennial run-off from Fennoscandia able to maintain marshes and levels of vegetation cover not encountered elsewhere in the region and to episodically drive major exorheic drainage systems into the Tethys Sea. Published climatic simulations are consistent with these observations, which indicate that Greenland, and particularly Fennoscandia, had precipitation levels higher than the regions to the south with a positive precipitation: evaporation balance.

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The Heron Clusterfieldsform part ofthe Eastern Trough Area Project,anintegrated development ofa totalofsevenfields. The mainreservoirwithinthe clusteristhe Triassic Skagerrak Formation (withundeveloped Jurassic PentlandF ormation inHeron andE gret). The fieldsarec lassified asHPHT reservoirs,withi nitial pressuresandtemperatureso f9300-12 900 psia nd300-350 8 Frespectively. Development ofthesefi elds initially assumed thatt he Skagerrak wouldb ewell connected andlikely to havea nactivea quifer. However, initialproduction from Heron in1998revealed adramatic pressuredecline, indicatingthatthe reservoirwasmore compartmentalized thananticipated, withlittleor no aquifer,necessitatingarevision to the reservoirmodel. The Skagerrak canbe subdivided into anupper,morechannel-dominated intervalandalower,poorerquality,more unconfined fluvialsection,bounded below bythe largely playaMarnock Shaleandabovebythe lacustrineHeron Shale.Re-description ofthe reservoirs uggested asettingd ominated byt erminals playdeposits,a rranged into cyclesbounded byah ierarchyo fshales,which formed laterally persistent,e ffectiveb arriers to verticalflow. Additionalperforations through the Skagerrak section encountered nearvirginpressuresandrestored production profilestoprognosis. Egretcameon-streamin1999 withasingleverticalwell andagainshowed arapid pressure decline, inthiscaseadditionallydueto fault compartmentalization. However,itwasobserved thatthe reservoir wasr e-chargingd uringshut-inp eriods,suggestingthatt he faults weretransmissiblewithsufficient pressure drawdown. Withe xperience from Heron andEgreto nv erticalcompartmentalization andfault transmissibility, Skuawasbroughton-streamin2001 withasinglehorizontalwell designed to intersectasmuch stratigraphyas possible.The Skagerrak reservoirinSkuaalso exhibits Egret-like rechargingacross faults. Overall the Skagerrak reservoirs inthe Heron Clusterappeartohavethe followingcommon characteristics:goodlateralconnectivity, but poor to zero verticalconnectivity; large faults become'leaky' withsufficient pressuredrawdown,andthere appears to be no aquifersupport. The short term production behaviour ofthesereservoirs isnot representativeof theirlonger-term behaviour,andinparticular,short-term well tests indicatealevelofcompartmentalization that doesnot materializeduringproduction.

The Sherwood Sandstone Group reservoir in the Wytch Farm field comprises a c. 150 m thick succession of arkosic sandstones deposited in a variety of fluvial, lacustrine and aeolian depositional systems. These systems show at least three orders of facies variability, which are interpreted to be the depositional response to climatic changes. These comprise a first-order evolutionary trend over the entire Sherwood Sandstone Group from perennial braidplain to ephemeral sheetflood systems to ephemeral lacustrine conditions. This trend culminated in deposition of the Mercia Mudstone Group, and reflects a long-term waning of sand supply and increasing 'flashiness' of the fluvial system. This trend is further subdivided into second-order cycles defined by five areally widespread floodplain and lacustrine deposits containing minimal development of fluvial sandstones. These represent widespread, episodic reductions in fluvial sediment supply and rising base level during more 'humid' climatic conditions. These horizons form the basis for the reservoir layering scheme. Each floodplain episode is increasingly more mud-rich upwards through the Sherwood section, and the sand-rich fluvial packages between become systematically more ephemeral in character. Third-order cycles are defined by thin (<2 m), but areally widespread floodplain and lacustrine horizons which are most readily identifiable in the upper half of the Sherwood section. The sandstones between these cycles are composed of aeolian and sheetflood deposits, but are incised by coarse-grained multistorey-multilateral channel deposits. The incisions are interpreted to be the result of fluvial erosion during dry climatic conditions when lake levels fell and the alluvial plain was devegetated. These incised fluvial deposits form the principal producing intervals in the upper part of the reservoir, particularly in the eastern part of the field. Higher frequency stratigraphic cycles are locally expressed by variations in ephemeral lake levels, palaeosol development and episodic development of wind-blown sand patches. At outcrop, the stratigraphically equivalent Otter Sandstone Formation (c. 100km to the west) shows comparable evolutionary patterns, albeit with a subtly different facies make-up. The recognition of a hierarchy of climatically driven cycles within the reservoir permits high-resolution correlation and the recognition of subtle, but important, changes in sandbody geometry and connectivity within successive cycles.Farm field, southern England.

This paper describes the nature and relative significance of stratigraphic and structural compartmentalization in dryland fluvial reservoirs using data drawn from the Heron Cluster (Heron, Egret and Skua) oil fields in the UK Central North Sea. The Triassic Skagerrak Formation reservoir in these fields was deposited in a variety of dryland terminal fluvial settings, ranging from relatively arid terminal splay and playa to more vegetated, channel-confined systems with associated floodplain and palustrine facies. Laterally extensive floodbasin shales punctuate this terminal fluvial architecture. Static and dynamic data indicate that these fields are compartmentalized: geochemical data indicate significant fluid variations both between wells and vertically within individual wells; material balance calculations suggest production from restricted connected volumes, locally from a subset of the range of oils present; and re-perforation across significant shale boundaries access undepleted reservoir with different fluid compositions. Lateral variations could be ascribed to prominent structuration within these fields, but in general these high net:gross reservoirs do not have a viable fault seal mechanism. Early (syn-halokinetic) grounding of Triassic 'pods' between salt swells during salt withdrawal has resulted in zones of intense faulting along the zone of contact of the pod and the underlying basement, and also on the flanks of pods as the margins collapsed under further salt withdrawal. This deformation occurred under relatively shallow burial depths and is largely expressed by disaggregation zones and phyllosilicate fault rocks. Fault property averaging algorithms (e.g. shale gouge ratio), indicate that the sands should communicate across the juxtapositions, implying that the fluids and pressures should equilibrate between reservoir sands. However, the stratigraphic differences across major shales in both fluid geochemistry and pressure caused by draw-down are preserved despite the presence of these faults. The preservation of stratigraphic compartments indicates that for these faults the deformation mechanism was probably dominated by clay smear, in which the shale-prone sequence was smeared down the fault planes without losing its coherence. This style of stratigraphic compartmentalization occurs across several shale-prone intervals that are correlatable across the region. In some cases these mark the boundaries to major changes in fluvial depositional character, provenance and floodplain drainage, suggesting an extrinsic control that led to shale packages defining consistent barriers in all the fields. Other shale barriers do not show major changes in depositional character and, although correlatable, appear to be the product of semiregional advance and retreat of the fluvial systems, possibly combined with nodal avulsion. In contrast to reservoirs deposited by large exorheic rivers, the terminal nature of these dryland fluvial systems appears to have resulted in the repeated interfingering of fluvial and floodb...