R. Bruce Ainsworth

Dynamic spatial and temporal prediction of changes in depositional processes on clastic shorelines: Toward improved subsurface uncertainty reduction and management

Existing classification schemes and models for clastic coastal depositional systems do not consider the potential amplifying or moderating effects of coastal morphology on depositional processes and do not provide a mechanism for the dynamic prediction of changes in coastal depositional style. A new process-based classification scheme based on the relative importance of primary, secondary, and tertiary processes is presented. This scheme permits a semiquantitative classification of clastic coastal depositional systems. In addition, it provides the basis for new models for clastic shorelines that convolve the effects of basin shape, coastal morphology, accommodation space, sediment supply, shoreline trajectory, and shelf width parameters on depositional processes. The end result is a marked improvement in the predictive capabilities of models. The models can describe and predict the likelihood of primary, secondary, and tertiary depositional processes acting in shoreline depositional environments via either a matrix or a decision tree approach. They are also dynamic in nature and can be applied to predict along-strike, updip, and downdip, or vertical changes in the dominance of depositional processes acting at any given location through geologic time. The key implications of these models are that given sets of known parameters, dominant and subordinate depositional processes or ranges of potential dominant and subordinate depositional processes acting at a coastline can be predicted. This provides an auditable methodology for determining reservoir modeling scenarios and reducing and managing the uncertainties in predictions of changes in clastic coastal depositional processes through time and space.

Where have all the lowstands gone? Evidence for attached lowstand systems tracts in the Western Interior of North America

Studies of outcrops of Upper Cretaceous strata from the Western Interior of Canada and the United States indicate that deposition during relative sea-level fall and lowstand can result in lowstand shorefaces that are attached to or in direct contact with the sand bodies of the immediately preceding sedimentary systems. This observation therefore represents an opposite end member from the Exxon sequence stratigraphic lowstand model, in which a detached lowstand shoreface develops during relative sea-level fall and lowstand. In this scenario, the lowstand shoreface is separated from the underlying shoreface by a zone that is bypassed by the net sediment load during sea-level fall and lowstand (sediment bypass zone). The sediments deposited during relative sea-level fall and lowstand (the lowstand systems tract; LST) can therefore be classified as detached (LSTd) or attached (LSTa). A major implication of this observation is that subsequent to a relative sea-level fall, it may not necessarily be correct to predict a detached lowstand shoreface lying basinward of the sand bodies of the underlying sedimentary system. Instead, the lowstand may have been deposited as a single depositional unit that is attached to or in contact with the sand bodies of the underlying deposits. 21 refs., 4 figs.

A hierarchical approach to architectural classification in marginal-marine systems: Bridging the gap between sedimentology and sequence stratigraphy

A new hierarchical architectural classification for clastic marginal-marine depositional systems is presented and illustrated with examples. In ancient rocks, the architectural scheme effectively integrates the scales of sedimentology (core, outcrop) and sequence stratigraphy (wireline-log correlation, reflection seismic). The classification also applies to modern sediments, which allows for direct comparison of architectural units between modern and ancient settings. In marginal-marine systems, the parasequence typically defines reservoir flow units. This classification addresses subparasequence scales of stratigraphy that commonly control fluid flow in these reservoirs. The scheme consists of seven types of architectural units that are placed on five architectural hierarchy levels: hierarchy level I: element (E) and element set (ES); hierarchy level II: element complex (EC) and element complex set (ECS); hierarchy level III: element complex assemblage (ECA); hierarchy level IV: element complex assemblage set (ECAS); and hierarchy level V: transgressive-regressive sequence (T-R sequence). Architectural units in levels I to III are further classified relative to dominant depositional processes (wave, tide, and fluvial) acting at the time of deposition. All architectural units are three-dimensional and can also be expressed in terms of plan-view and cross-sectional geometries. Architectural units can be linked using tree data structures by a set of familial relationships (parent-child, siblings, and cousins), which provides a novel mechanism for managing uncertainty in marginal-marine systems. Using a hierarchical scheme permits classification of different data types at the most appropriate architectural scale. The use of the classification is illustrated in ancient settings by an outcrop and subsurface example from the Campanian Bearpaw–Horseshoe Canyon Formations transition, Alberta, Canada, and in modern settings, by the Mitchell River Delta, northern Australia. The case studies illustrate how the new classification can be used across both modern and ancient systems, in complicated, mixed-process depositional environments.

Correlation Techniques, Perforation Strategies, and Recovery Factors: An Integrated 3-D Reservoir Modeling Study, Sirikit Field, Thailand

Production in the Sirikit oil field of the Phitsanulok basin, Thailand, commenced in 1982. The field has an STOIIP (stock tank oil initially in place) of approximately 800 MMbbl (million barrels). To date, more than 130 wells have been drilled and over 100 MMbbl produced. Reservoir management studies into optimizing recovery and identifying unswept oil volumes are ongoing so as to maximize the ventures profitability. This work includes 3-D (three-dimensional) static and dynamic modeling. This study is one such exercise whereby different geological correlation techniques and perforation strategies were analyzed for their potential impact on recovery factors. The Sirikit field and adjacent satellite fields are composed of predominantly lacustrine mouth-bar and fluvial deposits. Two correlation techniques, lithostratigraphy and chronostratigraphy, were employed to generate two 3-D reservoir architectures using the Shell proprietary 3-D static reservoir modeling system GEOCAP. Significantly different recovery factors are derived from 3-D dynamic flow simulation of these architectures (no vertical upscaling, 2 X 2 areal upscaling) within the Shell proprietary reservoir simulator MORES. When good-quality sands observed on well logs are perforated in both models, the lithostratigraphic correlation model results in an absolute 3% higher recovery factor than that of the chronostratigraphic correlation model. In relative terms, this represents a 43% higher recovery factor. This suggests that if a lithostratigraphic correlation model is assumed, then for this perforation strategy, the recovery factor would be overestimated by 3% absolute and 43% relative, if the chronostratigraphic interpretation represents the actual subsurface architecture. In addition, the architecture of the chronostratigraphic model results in the lateral correlation of the thicker upper mouth-bar sands with good-quality, thin-bedded toeset sands and shales (heterolithics). These thin-bedded sands are only marginally resolvable on wireline logs and have not generally been perforated in the Sirikit field. When these thin-bedded heterolithics are additionally perforated in the 3-D model, an absolute increase in recovery factor of 2% is observed over the same model that has only the upper mouth-bar sands perforated. In relative terms, this represents a 28% improvement in the recovery factor. This work confirms and partially quantifies the impact that the choice of mouth-bar correlation style can have on recovery factors as predicted by 3-D dynamic reservoir models. However, the chronostratigraphic model described in this paper represents an end-member connectivity scenario whereby sand-body continuity is relatively low due to continuous dipping shale barriers. Further work is required to quantify the recovery factors for the whole spectrum of potential chronostratigraphic model connectivity scenarios.

Sequence stratigraphic-based analysis of reservoir connectivity: influence of depositional architecture – a case study from a marginal marine depositional setting

This case study of the Sunrise and Troubadour fields (offshore northwest Australia) concentrates on the impact of primary depositional architecture on reservoir connectivity via a sequence stratigraphic-based, 3D reservoir modelling approach. The marginal marine reservoir is composed of fluvial-dominated and wave-dominated depositional environments. The succession is divided into six sequences and 12 systems tracts. Each systems tract is subdivided into parasequences which form the basic building blocks of the 3D model. The connectivity of sandbodies within each parasequence, systems tract and sequence were calculated when the models were palinspastically restored to a depositional datum. The findings indicate that depositional connectivity trends within a sequence stratigraphic framework are predictable. Connectivity trends can be related directly to depositional and stratigraphic trends and to position in the sequence stratigraphic hierarchy. Therefore, with a good understanding of depositional settings and high resolution sequence stratigraphic subdivision of strata, predictions of depositional connectivity trends at all stratigraphic hierarchical levels can be made. All connectivity trends at sequence and systems tract stratigraphic hierarchical levels remained the same when the area of the model was reduced by a factor of four and the volume of the model was reduced by truncation below the gas–water contact. Hence, the relationships between the stratigraphic trends and the connectivity trends for the thicker stratigraphic units can be said to be scale invariant. Three-dimensional reservoir models currently provide the best means of quantitatively assessing and predicting reservoir connectivity. However, in low fault density settings, initial screening of reservoir connectivity can be made using connectivity indicators calculated from simple parameters derived from well data such as ‘Thickness divided by Sand/Shale ratio’.

PREFERENTIAL ORIENTATION OF SHRIMP-GENERATED DIPLOCRATERION PARALLELUM AND THEIR RELIABILITY AS PALEOCURRENT INDICATORS

Abstract: Arthropods from the late Campanian (Late Cretaceous) of the Western Interior Seaway produced U-shaped Diplocraterion parallelum in mudstones along two closely spaced surfaces (10 cm apart), one of which corresponds to a maximum transgressive surface. Diplocraterion parallelum are widely distributed across both surfaces, with substantial variations in burrow orientations. Based on a comparison of paleocurrent indicators to burrow orientations, we demonstrate that D. parallelum are preferentially oriented parallel to the prevailing fair-weather wave propagation direction (wave-forced currents) that acted upon the colonized surfaces. There is an apparent maximum (34% above uniformity) preferential orientation of the burrows, attributed to the fact that wave-forced currents represent only one of several factors controlling shrimp burrowing behaviors. Based on this study, we propose that in the absence of paleocurrent data, Diplocraterion and other U-shaped burrows can be used to resolve flow directions, and that preferential burrow orientations ≥ 2% above uniformity are significant. However, it is noted that it is only possible to resolve 2-way flow directions (trend) from U-shaped burrows, as there is no way to determine vector directions.

Quantitative sequence stratigraphy

Sequence stratigraphy based on wire-line logs, cores, and outcrops is entering its fourth decade of mainstream usage in industry and academia. The technique has proved to be an invaluable tool for improving stratigraphic analyses in both clastic and carbonate settings. Here we present a simple quantitative technique to support sequence stratigraphic interpretations in clastic shallow marine systems. The technique uses two pieces of data that are readily available from every subsurface field or outcrop study: (1) parasequence thickness (T) and (2) parasequence sandstone fraction (SF). The key assumptions are that parasequence thickness can be used as a proxy for accommodation at the time of deposition and parasequence sandstone fraction can be used as a proxy for sediment supply. This means that quantitative proxies for rates of accommodation development and sediment supply can be acquired from wire-line logs, cores, and outcrop data. Vertical trends in parasequence thickness divided by sandstone fraction (T/SF) approximate trends expected in systems tracts for changes in ratios of rate of accommodation development to rate of sediment supply. The technique, termed “TSF analysis,” can also be applied at lower-order sequence and composite sequence scales. It provides a quantitative and objective methodology for determining rank and order of sequence stratigraphic surfaces and units. Absolute T/SF values can be used to determine shoreline, stacked shoreline, and shelf-margin trajectories. Four case studies are presented, which demonstrate the robustness of the technique across a range of different data sets. Implications and potential future applications of TSF analyses are discussed.