Changes

==True stratigraphic thickness==

==True stratigraphic thickness==

''True stratigraphic thickness'' (TST) is the thickness of a stratigraphic unit measured in the direction perpendicular to the bedding planes of the unit (Figure 2). It is a critical measure for understanding both the structural and stratigraphic development of a field. The true stratigraphic thickness is derived from the true vertical depths by the following equation:

 

 

''True stratigraphic thickness'' (TST) is the thickness of a stratigraphic unit measured in the direction perpendicular to the bedding planes of the unit ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig2.png|Figure 2]]). It is a critical measure for understanding both the structural and stratigraphic development of a field. The true stratigraphic thickness is derived from the true vertical depths by the following equation:

:<math>\mbox{TST}  = (\mbox{TVD}_{b} - \mbox{TVD}_{t}) \times (\cos \delta') - [(\mbox{NSD}_{b} - \mbox{NSD}_{t})^{2} + \, (\mbox{EWD}_{b} - \mbox{EWD}_{t})^{2}]^{1/2} \times \sin \delta')</math>

:<math>\mbox{TST}  = (\mbox{TVD}_{b} - \mbox{TVD}_{t}) \times (\cos \delta') - [(\mbox{NSD}_{b} - \mbox{NSD}_{t})^{2} + \, (\mbox{EWD}_{b} - \mbox{EWD}_{t})^{2}]^{1/2} \times \sin \delta')</math>

[[file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig6.png|thumb|{{figure number|6}}Relationship between true dip of a planar surface and apparent dip of that surface in a plane at an angle of ε to the dip direction.]]

In this equation, δ′ indicates the apparent dip of the bed in the direction of the horizontal displacement ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig6.png|Figure 6]]), which is written as

 

In this equation, δ′ indicates the apparent dip of the bed in the direction of the horizontal displacement (Figure 6), which is written as

:<math>\delta' = \tan^{-1} [\tan \delta \cos (\beta - \varepsilon)]</math>

:<math>\delta' = \tan^{-1} [\tan \delta \cos (\beta - \varepsilon)]</math>

It is important to note the sign convention for the two angles α and δ′. The deviation is measured from the vertical and is positive, whereas the dip is measured from the horizontal and is positive if it is in the same direction as the deviation and negative if the dip is opposite to the deviation.

It is important to note the sign convention for the two angles α and δ′. The deviation is measured from the vertical and is positive, whereas the dip is measured from the horizontal and is positive if it is in the same direction as the deviation and negative if the dip is opposite to the deviation.

An assumption made here is that the dip of the top and bottom surfaces is essentially the same. The more closely the wellbore direction approximates the dip direction, the more sensitive the thickness calculation will be to stratigraphic thickness changes (see Figure 7a). The assumption is also violated if the well traverses a zone of strong curvature in the rock such that dip changes rapidly (Figure 7b). Such changes can be corrected for if sufficient data are available, but are commonly too small to be of significance.

An assumption made here is that the dip of the top and bottom surfaces is essentially the same. The more closely the wellbore direction approximates the dip direction, the more sensitive the thickness calculation will be to stratigraphic thickness changes (see [[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig7.png|Figure 7a]]). The assumption is also violated if the well traverses a zone of strong curvature in the rock such that dip changes rapidly ([[:file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig7.png|Figure 7b]]). Such changes can be corrected for if sufficient data are available, but are commonly too small to be of significance.

 

[[file:conversion-of-well-log-data-to-subsurface-stratigraphic-and-structural-information_fig7.png|thumb|{{figure number|7}}Conditions under which true stratigraphic thickness calculated according to its equation will be in error. (a) Variation in stratigraphic thickness is large in relation to the downdip deviation of the well path. (b) Change in dip produces an error in estimation of the true stratigraphic thickness.]]

To calculate a TST requires survey information as well as some measure of the dip of the beds. Dip can be derived from dipmeter logs (with some caution) (see “[[Dipmeters]]”) or from maps of geological structure. In some instances, TST maps reveal anomalies in well correlation, resulting in iterative refinement of structural and stratigraphic models. It must be recognized that, where folding occurs, stratigraphic thickness trends on maps of true stratigraphic thickness will be distorted by compression.

To calculate a TST requires survey information as well as some measure of the dip of the beds. Dip can be derived from dipmeter logs (with some caution) (see “[[Dipmeters]]”) or from maps of geological structure. In some instances, TST maps reveal anomalies in well correlation, resulting in iterative refinement of structural and stratigraphic models. It must be recognized that, where folding occurs, stratigraphic thickness trends on maps of true stratigraphic thickness will be distorted by compression.