Author: Mateus da Costa
Department of Petroleum Engineering, University of Stavanger, Norway
Abstract:
Sequence Stratigraphy is a modern discipline of sedimentary geology that deals with
stratigraphy in a broad sense. It investigates the relations between sedimentations, unconformities, and or conformities which are generated in a basin fill history. The important key unit in sequence stratigraphy is the stratigraphic sequence, because it retains a succession of strata which can be delineated chronologically and spatially to generates some properties that can define total geological history with respect to certain events and processes, Hence sequence stratigraphy deals with sedimentary processes, factors that control sedimentations, the environmental depositions and sedimentary rocks’ characteristics. Based on previous publications, this paper is going discuss about the factors that control the sequence stratigraphy, shoreline shifts, system tracts and sequence stratigraphic models, and finally it will review some applications of sequence stratigraphy, specifically in petroleum explorations industries.
Introduction
Sequence Stratigraphy is a modern discipline of sedimentary geology that studies about stratigraphy in a broad sense. It investigates the relations between sedimentations, unconformities, and or conformities which are generated in a basin fill history. The important key unit in sequence stratigraphy is to retain a succession of strata related which can be delineated chronologically and spatially, and generates some properties that can define total geological history with respect to certain events and processes, (Gradstein et al, 1998). Hence sequence stratigraphy also deals with sedimentary processes, factors that control sedimentations, the environmental depositions and sedimentary rocks’ characteristics. According to several publications, (Emery et al, 1996, Gradstein et al, 1998, Catuneanu, 2002, El-ghali, 2004, Catuneanu, 2005), term sequence stratigraphy was introduced and developed in late 1970s along side with the emerging of the idea of seismic stratigraphy. The term sequence itself, however, has been introduced by early workers several decades prior to the birth of the seismic stratigraphy. During early 20th centuries right up the late 1950s the term sequence was used to designate stratigraphic units bounded by unconformities, and to emphasize the importance of tectonism in the generation of sequence and bounding unconformities, (Catuneau, 2002). Based on previous publications, this paper is going discuss about the factors that control the sequence stratigraphy, shoreline shifts, sequence tracts and sequence stratigraphic models, and the applications of sequence stratigraphy, specifically in petroleum explorations industries.
1. Controls on sequence stratigraphy
There are several factors that influenced the successions of sequence stratigraphic units. Catuneanu et al (2005), identify six main factors that control sequence stratigraphic succession. These six factors are eustasy and tectonism, climate, sediment supply, basin physiography and environmental energy. The interaction between tectonism, eustasy and rate of sediment supply will influence the accommodation or available space for sediment accumulation. Moreover, basin architecture which is mainly controlled by different types of tectonic margins will have a high influence on uplift and subsidence that control base level changes besides the rate of sediment supply. Hence, basin architecture has an important control on accommodation at the first stage. It also contributes to the change in sea level and local climates due to uplift and subsidence. Climatic changes and environmental energy increase the erosion which predominantly in fluvial environments. Rising and falling of sea level changes also have a big impact on the accommodation and the shifts of shore. Figure 1.1 shows the development processes of sequence stratigraphic successions through time and in response to different controlling factors.
Figure 1.1 from Emery et al (1996) shows sequence stratigraphy succession changes from Transgressive.
When the rates of sediment supplies outpace accommodation creation, shoreline will shift seaward causing the progradation of facies belt. On the other hand, when the rate accommodation creation and rising sea level is faster the rate supply, it will result in sediment starvation in basin fill which will also lead to the shoreline to shift landward and retrogradation of facies belts. The combination of tectonic loading and subsidence due higher rate sediment supply will cause base level to fall; hence it will increase more accommodation. This will overturn the regression to transgression.
2. Base level changes and Shoreline shifts
Base level represents the surface of equilibrium between erosion and deposition. It represents the highest level where sedimentary succession can be built. Catuneanu (2002) defines that base level is a global reference surface points to which continental denudation and marine aggradation proceed. In a very simplistic manner, base level can be analogized as sea level, although in reality base level locates below sea level due to erosions that caused by waves and marine currents. Base level surface is dynamic and always varies with rising and falling of sea level. Base level tends to adjust with continental denudations, though at some areas where sediment loads exceeding the transport capacities the processes of nonmarine aggradation may still take place. At fluvial system tends to form a longitudinal equilibrium profile by transporting its sediment loads without aggradation or degradation of channels. When the elevations of sediment source areas change due to denudation, subsidence/uplifts Rivers may start a new equilibrium surface, which may form above or below land surface and merge with the marine base level equilibrium.
2.1 Transgression
As it has been discussed earlier, transgression takes place when the accommodation is created much faster than the rate of sediment supply and accumulation, hence there is sediment starvation in basin fill. As result base level rise and outpace the sediment rates in at the shoreline. The rise of the base level causes coastal aggradation. During transgression the shoreline shifts gradually and in landward direction causing the facies belts to retrograde. At this stage the sedimentation and erosion determines the types of coastal transgressive. The aggradation predominant coastline will preserve estuarine whereas the erosion predominant coastline will preserve unconformities in the non-marine part of the basin. Regardless of nature of the coastline, the wave revinemnt surface is onlapped by transgressive shallow marine that can provide a good understanding of the transgressive nature of subareal unconformities.
2.2 Regression
Regression occurs when rate of sediment supply outpaces the rate sediment creation and the base level fall। There are two types of regressions; forced Regressions and normal regression. Forced regressions occur when base level and forcing the shore line to shift landward. It is normally characterised by high sediment supply into the deep marine environment. This because of the additional erosion and the lack of accommodation the fluvial and shallow marine/shore face environment hence the sediments bypass the two settings. Normal regression in the early and late stages of base level rise, when the rate of accumulation is higher than the rate of base level rise. Figure 2.1 shows the progradation, retrogadation and aggradation of facies belt due to changes in base level.
Figure 2. from Gradsten (1998) ( xxon model) shows facies distribution due in different sequence stratigraphic sequences.
3. System tracts and Sequence models
3.1. System tracts
The term system tract is used to define a links between contemporaneous depositional systems that form from as a sub-divisional sequence. In general system tracts categorised into four parts (Emery at al, 1996, Gradstein, 1998, Catuenenu 2002); Transgsgressive system tract (TST), Highstand system tract (HST), Lowstand system tract (LST), and regressive system tract (RST). HST is bounded by maximum flooding surface at the and at the top which, subareal unconformities, regressive marine erosion and basal forced regression. HST is associated with base level rise. LST is associated with base level fall and is characterised by shallow marine that deposits with prograding and onlapping stacking patterns equivalent to deep marine and submarine fan.
on model) shows the down dip system tracts idealised for passive margin shelf succession.
TST is bounded by maximum regressive surface at the base and by the maximum flooding surface at the top. It is formed when sediment rates is outpaced by base level rise. TST is characterised by retrogradational stacking pattern that cause fining upward profiles within both marine and non-marine stratigraphic successions. RST composed all strata that accumulate during the shore regression. This means that it consists of both HST and LST. It is bounded by maximum flooding surface at the top and defined by progradational facies belts of both marine and nonmarine successions. Figure 3.1.a and 3.1.b. of the Exxon models summarise the whole system tracts in both 2-D and 3-D views. It should be noted that in figure
3.2 Sequence models
According to Emery (1996), Gradstein (1998) and Catuneanu (2002), the sequence stratigraphic model can be categorised into two main groups; the depositional sequence which defines the sequence boundaries based on the base level curves and the genetic stratigraphic – Transgressive regressive sequence that define the sequence boundaries relative to the Transgressive-Regressive curves. The advantage of this model is that its correlative conformity is independent because it is based on the base level curves, but it can be used to visualise the high resolution data such well logs, small average size of outcrops and even seismic in data with intervals of one to ten meters. The genetic sequence model however, has maximum flooding surface that can be used to map across the basin but since it includes subaerial unconformity within the sequences, it may also include the unrelated genetic strata into the genetic package. The transgressive-regressive model provide and alternative way of grouping strata sequences by overcoming the disadvantages the depositional sequence when it comes to visualising high resolutions data and avoiding the inclusion of the unrelated genetic strata during the genetic strata packaging. But, it face problem when we try to employ this model to delineate deep marine strata sequences.
4. Applications of Sequence: specifically for Petroleum Explorations
As it has been discussed earlier, sequence stratigraphy was developed during time of the emerging of use of seismic stratigraphy. Although the use of sequence stratigraphy can be applied widely in many different field of geology, however, in this paper our discussion will only be limited into the applications in sequence in petroleum exploration industries and coals. In the real life of Petroleum world sequence stratigraphy plays a very important role in improving ones understanding about the basin fill history, facies distribution in both reservoir and source rocks as well as the sealing rocks. Sequence stratigraphy enhances our understanding about core analysis and well log correlations. In coal mining industries, sequence stratigraphy will improve our understanding about to where the coal layers and peat layers extend. Coal and peats tend to be associated with the HST, because of the increase in accommodation during regression, Koster et al, (2000). By studying sequence stratigraphy successions on rock outcrop, cores will and by correlating both seismic stratigraphic and well data will help us in reconstructing the palaeo-environment and basin fill history. Sequence stratigraphy also helps us in in understanding the reservoir, source and cap rocks distribution, Koster, (2000).
5. Conclusion
In conclusion it can be said that although in general Catuneanu (2005) categorises the factors that control sequence stratigraphy are six, however, due counter act influences on each other, we can mainly the six factors into three; tectonism and eustasy, accommodation space and rate of sediment supply. The basin architecture is mainly depended on the tectonism, the climate and environmental energy are mainly depended on tectonism and eustasy and the interactions between tectonism and eustasy. For sequence modelling, although the two main model are contradicts, however they can be employed in different case whenever are needed. The depositional model is more suitable for modelling the low resolution data such deep marine strata sequences, whereas the genetic stratigraphic and the transgressive-regressive model can be suitably used for visualising high resolution strata sequence.
References:
• Catuneanu, O., 2002. Sequence Stratigraphy of Clastic Systems: Concepts, Merits and Pitfalls. Journal of African Earth Sciences 35, 1-43.
• Catuneanu, O, Martins-Neto,M.A., Eriksson,P.G. 2005. Precambrian Sequence Stratigraphy. Sedimentary Geology 176, 67-95.
• El-ghali, M.A.K 2005. Depositional Environment and Sequence Stratigraphy of Paralic Glacial, Paraglacial and Postglacial Upper Ordovician Siliclastic Deposits in the Murzuq Basin, SW Lybia. Sedimentray Geology 177, 145-173.
• Emery, D., and Myers,K.J., 1996. Sequence Stratigraphy. Blackwell Science Ltd. Australia
• Gradstein,F.M., Sandvik,K.O., Milton, N.J. 1998. Sequence Stratigraphy Concepts and Applications. Norwegian Petroleum Society (NPF), Special Publication No. 8.
• Karst, E.C., VanderZwaan, G.J., Jorisess, F.J. 2000. International Journal Coal Geology 43, 13-26.
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