Alluvial Architecture and Base-level Fluctuation

It is difficult to make a sequence stratigraphic analysis or apply sequence stratigraphic concepts to such a short stratigraphic column within a small study area. The regional dip of the strata also makes the correlation somewhat difficult and limits the outcrop extent of the architectural elements. Moreover, the facies do not change greatly as virtually all sandstone bodies contain tidal signature, though to some different extent.

Nevertheless, there have been recorded 4 marine flooding surfaces and changes in the architectural style within the stratigraphic column. The changes in alluvial architecture may be interpreted to represent changes in the base-level rise or fall and the rate of these changes, which is an important control on accommodation space created (Shanley and McCabe, 1993). Candidates for sequence boundaries in an alluvial setting can be unconformable surfaces that have experienced erosion, incision and long term subaerial exposure, as for example regional paleosols. Additionally, a basinward shift in facies, often represented by an abrupt contrast in the facies and architectural style, should be expected at a sequence

boundary (Shanley and McCabe, 1993). The modest changes in facies across the

stratigraphic column may suggest only small changes in base-level, with the largest changes in base-level represented by the flooding surfaces. Any of these flooding surfaces are candidates for a maximum flooding surface.

Sequence boundaries start to form at the initial stage of base-level fall and may continue to be formed during early slow base-level rise (Fig. 9.2 A). Simultaneously, erosion and sediment bypass may occur together with the formation of a prograding shoreface deposit (Zaitlin et al. 1994; Posamentier and Allen, 1999). Candidates for sequence boundaries in the study area are the lowermost paleosol (0 meters), the paleosol between the sections 1 and 2 and the paleosol between sections 2 and 3. All of these paleosols are difficult to trace and may not be regional, but are candidates since the alluvial architecture and sometimes the facies change across these surfaces. Thus several sequences appear to be present with several candidates for sequence boundaries and maximum flooding surfaces.

Fig.9.2: Illustration of relationship between fluvial architecture and base-level changes. A) Base-level fall to stillstand give rise to fluvial incision, sequence boundary development and deposition of a prograding lowstand wedge. B) Reduced rated of base-level fall to initial base-level rise result in aggradation and amalgamated fluvial deposits. C) Increased rates of base-level rise lead to retrograding shoreface paraseuences, decreasing sand-mud ratio. This is the equivalent to the transgressive system tracts which is overlain by a maximum flooding surface that is temporally equivalent to tidally-influenced fluvial deposits. From Shanley and McCabe (1993).

Sandstone proportions in a fluvial depositional environment largely rely on the

accommodation space for sediments and the supply of sediments. Base-level fluctuation is an important factor which controls the accommodation space, though other factors, like

tectonics, are also essential controls many places. In general, low sandstone proportion is associated with high accommodation space and little incision during rising and high base-level. High sandstone proportion is associated with incision and low accommodation space during low base-level or high tectonic subsidence (Bridge et al. 2000; Posamentier and Allen, 1999; Miall, 1996). Estuarine development may occur during sea-level rise after this stage.

As the base-level raises the fluvial deposits aggrade together with the aggrading/retrograding phase of the shoreface. At this point the channels switch rapidly, consuming floodplain fines by lateral erosion, resulting in amalgamation of channel sandstone bodies (Fig 9.2 B), as suggested in the model of (Shanley and McCabe, 1993), and high sand:gross ratio (Laure and Hovadik, 2006). The high degree of lateral stacking of elements as well as vertical stacking of bars and thick and extensive levee complex indicate an aggradational setting of stable or slowly rising relative sea-level (Komatsubara, 2004).

There is a general upward increase in sandstone body proportion and thickness in the stratigraphic column. Section 0 may be the equivalent stage of the marine lowstand systems tracts bounded below by a candidate sequence boundary, representing initial base-level rise with fluvial deposits. Section 1 has a majority of isolated sandstone units and high proportion of floodplain fines which can represent a rapid rise in base-level and increase in A/S, or the alluvial equivalent to highstand system tract corresponding to an aggrading shoreface (Shanley and McCabe, 1993). Section 2 show signs of low or initial base-level rise with a stable channel generating large point bar deposits and local paleosol development. Section 3 shows an aggradational setting with bars and levee complex which may indicate a stable or slowly rising base-level. Section 4 may record a decreasing rate in base-level rise where the lower part contains some isolated sandstone units, but also a small levee complex, with higher degree of amalgamation towards the top.

10 HETEROGENEITY

To determine the reservoir heterogeneity is significant in relevance to the oil recovery expected from the reservoir as the heterogeneity has a major impact on the reservoir

properties of porosity and permeability. Heterogeneity occurs at several levels of scale and as different types of heterogeneity which commonly has a directional variation (Weber, 1986).

In an unfaulted succession, as the one observed in the Louriñha Formation, heterogeneity types can be observed at: 1) boundaries between genetic units, where boundaries between channel fill and adjacent elements of e.g. floodplain or crevasse splays will have a higher directional heterogeneity e.g. perpendicular to the channel axis than along the axis on this scale; 2 & 3) permeability zones and barriers (baffles) within a genetic unit, like a

multistorey channel fill and IHS, which will most likely have a higher heterogeneity along the vertical axis or obliquely within the element; 4) laminations and cross-stratification, like mud drapes in cross-stratified sand and mudstone-sandstone laminae, which will have a higher heterogeneity along the vertical axis or obliquely on the facies scale; 5) and microscopic heterogeneity on the textural level of the grains and mineral composition.

The types of heterogeneity mentioned above are only 5 of a total of 7 which are included in Weber’s (1986) classification scheme of reservoir heterogeneity types. Faulting and

fracturing are the two remaining types of reservoir heterogeneity and are not included as they are not largely present in the Louriñha Formation. In general, with the exceptance for faulting and fracturing, the 5 types of heterogeneity can be said to be an identification of depositional environments as the porosity and permeability properties are a function of the depositional environment and sedimentary facies which has later been modified by its diagenetic history (Nystuen and Fält, 1995; Weber, 1986).

There are several different ways of organizing the different levels of heterogeneity in a sedimentary succession. Aigner et al. (1998) suggested a heterogenic hierarchy which includes 5 levels reaching from microscale (first-order) up to both autogenic (megascale, fourth-order) and allogenic (gigascale, fifth-order) base-level changes. Jiao (2005) proposed a less detailed division of the heterogenic hierarchy where megascale is the upper

heterogenic level and here represents the depositional environment of architectural elements, which is included in the macroscale level in the hierarchical system of Aigner et al. (1998).

Miall (1996) introduced a fourth-order hierarchical system, quite similar to the system of Aigner et al. (1998), which will be applied here:

1) Megascale heterogeneity, which is caused by the architectural style and base-level fluctuations and includes the variations across sedimentary units within the entire basin.

2) Macroscale heterogeneity, which is caused by the various architectural elements and variability associated with the deposition of these;

3) Mesoscale heterogeneity, which is caused by depositional processes, bedding units sedimentary structures;

4) Microscale heterogeneity, which is caused by particle properties like grain size, texture and diagenetic processes, and are responsible for the petrophysical properties (porosity and permeability) of the rocks

(Fig. 10.1).

Fig. 10.1: Levels of heterogeneity; from Megascale to microscale. Modified form Weber (1986).