Exploration Insights September 2019 - Page 26

26 | Halliburton Landmark Exploration Handbook | 27 A Stratigraphic Fill B Tectonostratigraphic Phases © 2019 Halliburton B A © 2019 Halliburton Crustal extension Mobile unit Passive subsidence © 2019 Halliburton © 2019 Halliburton Figure 5 > Stratigraphy has a close relationship to tectonic regime. In this example, the phase of crustal extension (orange) relates to the formation of graben filled with typical syn-rift stratigraphy, while the passive margin phases (green) comprise classic marine stratigraphy transitioning from shallow to deep facies there periods of hinterland uplift that might have delivered coarse clastics into the basin and created potential reservoirs? Paleoclimate can be used to determine when the hinterland experienced the most intense weathering and where permeable carbonates may have been deposited. Accurately defining periods of rifting, uplift, and tectonic quiescence in a basin is important, in order to provide a thorough understanding of the development, distribution, and preservation of key play elements. Evaluating Play Elements For accurate play prediction, it is vital to understand the stratigraphic fill of a basin, as discussed above. Once this is established, it is possible to consider where the various play elements could exist within the tectono-stratigraphic framework. Play cross sections allow these concepts to be illustrated through the depiction of key play elements: source rocks; reservoirs; seals; and traps (Figure 6). Play cross sections depict the spatial distribution of likely source rocks, highlighting whether they are laterally extensive, or perhaps restricted to graben, and also indicate the approximate burial depth. For reservoirs, we can assess the composition of packages from wells and use this to predict the reservoir composition, from more proximal to distal areas. For seals, the play cross section can highlight areas where the thickness of sealing units, or the intensity of faulting and folding, may be favorable or detrimental to seal capacity. Through the representation of the structural framework of the area, the play cross section can identify structures that configure trap geometries, such as tilted fault blocks or anticlines. Through the stratigraphic fill, it can highlight more subtle trapping mechanisms, including stratigraphic traps where reservoir facies pinch out in sealing facies. Figure 6 > Predicting play elements. A) Lacustrine source rocks within syn-rift graben. B) Deep-marine reservoir prediction in a pro-delta setting. Well data help to constrain the occurrence of coarse clastics, but using sequence stratigraphy and understanding the tectono-stratigraphy of the basin can be predictive of other coarse clastics in the deeper basin. Identifying play elements allows us to assess the potential of certain plays, by considering key criteria that affect the ability of a system to produce and store hydrocarbons, including charge, composition, trapping mechanism, and seal capability. Defining the Oil and Gas Window To assess the play potential of the area, it is important to consider whether possible source rocks are likely to generate hydrocarbons, and what the likely hydrocarbon phase will be. Important thresholds to define on a play cross section are the oil generation threshold (OGT), base oil window (BOW), base biogenic gas, and thermos-sulphate reduction threshold — although not all will be relevant in every situation. It is also important to consider whether the source rocks are generating at present-day, typically indicating they are at maximum burial, and in most (but not all) instances, maximum overburden. Hydrocarbon phase thresholds can be assessed using data, including temperature data (bottom-hole temperatures from wells and geothermal gradients), maturity data (vitrinite reflectance and spore color index), and an understanding of the burial history of the basin. An understanding of the structures and lithologies present in the area, which may affect heat flow, such as salt and volcanics (Figure 7), is also required. Defining Plays Plays need to be defined in a standard manner to enable the comparison of individual play elements or plays as a whole. Classification can be made based on a number of different factors, “For accurate play prediction, it is vital to understand the stratigraphic fill of a basin”