Exploration Insights September 2019 | Page 24

24 | Halliburton Landmark A Passive Margin Basin Fold and Thrust Belt Rift Basin Intracratonic Sag Basin © 2019 Halliburton © 2019 Halliburton B Figure 3 > The tectonic regime has a direct impact on the structural framework of the basin and, consequently, affects where play elements might be deposited and preserved. The positioning of the section transect must also be considered. Is the section representative of the basin as a whole or is it placed to intersect certain features, such as a rift basin that has syn-rift source rock potential or a structural feature that provides potential trap geometries? In most cases, it is advisable to cover the deepest stratigraphy (to capture the widest stratigraphic interval) and focus the section where data are available to validate and constrain insight. Structural Framework At the most basic level, a play cross section summarizes the tectono-stratigraphic evolution of the study area and helps to define the type of basin (extensional, compressional, or transtensional) and, therefore, what structural style is expected by the geologist. For example, a transect through a sag basin is structurally very different from a passive margin or foreland basin (Figure 3), with implications for the stratigraphic response and prospectivity alike (Gluyas and Swarbrick, 2004). It is important to note whether the seismic dataset is represented in two-way travel time or has been converted into depth. The representation of units in two-way travel time is not directly proportional to true stratigraphic thickness due to seismic velocity changes. For example, velocities tend to increase with depth due to compaction, so deeply buried beds may appear thinner than their true thicknesses. Seismic velocity changes through the section may also create false geometries, such as the pull up effect seen below salt caused by its low velocity. If the line has been depth-converted, it is important to consider what model was applied, in order to test the validity of the depth information provided. Integration of tectonic element maps, surface geology maps, depth maps, isopach maps, and 3D depth data also constrains the structural framework. Wells and well tops can be used, but bear in mind that wells are preferentially drilled on highs and, therefore, may not be representative of deeper stratigraphy. In frontier basins, where data are sparse, a basic structural framework can be gained from an understanding of the plate-scale evolution of the region (Figure 4). Tectono-stratigraphic phases need to be identified, in order to begin to understand the fill of any basin. These phases will vary depending on the tectonic regime. In a rift-basin, they can broadly be grouped as pre-rift, syn-rift, and post-rift elements; and if relevant, any other main tectonic phases can also be identified, such as inversion. They can be defined from geometries identified on seismic and in cross sections, and typically have a predictable stratigraphic response. Infilling the Stratigraphy Filling in the stratigraphy of the section is largely accomplished using well and outcrop data. These provide specific stratigraphic data, such as the depths to particular horizons Foreland Basin Foreland Basin Exploration Handbook | 25 © 2019 Halliburton Continental sediments Fine-grained deep- water siliciclastics (marine) Sabkha evaporites Coarse-grained shallow- water siliciclastics Fine-grained siliciclastics (lacustrine) Salina/saltern evaporites Volcanics Coarse-grained deep- water siliciclastics Shallow-water carbonates Deep-water evaporites Basement Fine-grained shallow- water siliciclastics (marine) Deep-water carbonates Biosiliceous ooze/chert Oceanic crust Organic-rich sediments Figure 4 > Different tectonic evolutions produce different basin types, in terms of their structure and stratigraphic fill. The difference is clear, looking at: A) a fold and thrust belt, where thick-skinned and thin-skinned thrust packages affect the distribution of stratigraphy; compared with B) a tectonically simple foreland basin. or play elements, thicknesses of units, and lithofacies. Data to infill the remaining tectono-stratigraphic packages are derived from chronostratigraphic charts, GDE maps, and stratigraphic information in text and facies maps. The principles of sequence stratigraphy, (described in the April edition of the Exploration Handbook), can be employed to make predictions of the likely stratigraphic response to base level change away from areas of data constraint. The Neftex Insights portfolio includes a proprietary sequence stratigraphic model that allows predictions to be made at a high degree of temporal resolution. In the absence of well data, we can use our understanding of the tectonic regimes to give insight into the stratigraphic fill associated with each tectonic phase. Certain tectonic regimes give rise to predictable stratigraphic responses that result in the deposition of certain petroleum system elements. For example, syn-rift tectonics and facies vary from those deposited once a post-rift tectonic regime is established. Syn-rift facies are often characterized at their commencement by continental and lacustrine sediments. If rifting continues and a passive margin is established, shallow- marine facies will be deposited (Figure 5). The Neftex Sequence Stratigraphic Model can be overlain on top of the major tectono- stratigraphic phases to give a higher degree of temporal resolution when considering different events. Global and regional events can also be used to influence the interpreted stratigraphy. For example, could oceanic anoxic events have resulted in organic enrichment in the area, and when were “At the most basic level, a play cross section summarizes the tectono- stratigraphic evolution of the study area and helps to define the type of basin”