Changes

A number of surfaces are typically mapped during reservoir development to show closure and other limits to reservoir production. Maps of top of pay and bottom of pay can also be “subtracted” to determine pay thickness.

A number of surfaces are typically mapped during reservoir development to show closure and other limits to reservoir production. Maps of top of pay and bottom of pay can also be “subtracted” to determine pay thickness.

===Structure===

===Structure===

Structure maps show lines of equal elevation or depth for a selected marker horizon (Figure 1) (see [[Evaluating structurally complex reservoirs]]). Mean sea level is a useful reference datum. Commonly contoured horizons are top of zone or top of net pay. Control points are provided by surveyed wells and can be supplemented by [[seismic interpretation]]s, especially offshore. In highly developed fields, typically onshore, sufficient well control might exist to allow geostatistical interpolation between control points (see Part 8).

Structure maps show lines of equal elevation or depth for a selected marker horizon ([[:file:subsurface-maps_fig1.png|Figure 1]]) (see [[Evaluating structurally complex reservoirs]]). Mean sea level is a useful reference datum. Commonly contoured horizons are top of zone or top of net pay. Control points are provided by surveyed wells and can be supplemented by [[seismic interpretation]]s, especially offshore. In highly developed fields, typically onshore, sufficient well control might exist to allow geostatistical interpolation between control points (see Part 8).

[[file:subsurface-maps_fig1.png|thumb|{{figure number|1}}Structure map of the top of the T5 marker, Frlo Formation, Brazoria County, Texas. (After <ref name=pt06r12>Bebout, D. G., Loucks, R. G., Gregory, A. R., 1978, Frio Sandstone reservoirs in the deep subsurface along the Texas Gulf coast—their potential for production of geopressured geothermal energy: The Univ. of Texas Bureau of Economic Geology Report of Investigations, n. 91, 93 p.</ref>.)]]

[[file:subsurface-maps_fig2.png|thumb|{{figure number|2}}Structure of the base of the Humber unconformity (top of the Brent Group), Dunlin field, U.K. Northern North Sea mapped with 1979 and 1989 vintage data. Contours are marked in ft subsea × 100; contour interval is 100 ft. (From <ref name=pt06r16>Braithwaite, C. I. M., Marshall, J. D., Holland, T. C., 1989, Improving recovery from the Dunlin field, U., K. Northern North Sea, in Formation Evaluation and Reservoir Geology: 64th Annual Technical Conference of the Society of Petroleum Engineers, San Antonio, TX, Oct. 8–11, SPE 19878, 18 p.</ref>.)]]

===Fault planes===

===Fault planes===

Faults are special surfaces whose traces will show on structure contour maps (Figures 1 and 2). Faults form bounding surfaces for some reservoirs, and sufficient well control might exist to contour map the fault surface itself. Projections of subsurface data into the plane of the fault are also useful “maps” for reservoir development, but are more appropriately described as cross sections. (For details of construction of fault plane maps, see the chapter on “Conversion of Well Log Data to Subsurface Stratigraphic and Structural Information” in Part 6.)

Faults are special surfaces whose traces will show on structure contour maps ([[:file:subsurface-maps_fig1.png|Figures 1]] and [[:file:subsurface-maps_fig2.png|2]]). Faults form bounding surfaces for some reservoirs, and sufficient well control might exist to contour map the fault surface itself. Projections of subsurface data into the plane of the fault are also useful “maps” for reservoir development, but are more appropriately described as cross sections. (For details of construction of fault plane maps, see the chapter on “Conversion of Well Log Data to Subsurface Stratigraphic and Structural Information” in Part 6.)

 

===Unconformities and subcrops===

===Unconformities and subcrops===

Surfaces of unconformity can be especially useful marker horizons for structure contour mapping (Figure 2). In many fields, unconformities are the location of sealing shales and/or source rocks above reservoir pay. Subcrop maps, traces of productive zones, barriers, or marker horizons mapped on the unconformity surface are invaluable for planning well placement and for reservoir development.

Surfaces of unconformity can be especially useful marker horizons for structure contour mapping ([[:file:subsurface-maps_fig2.png|Figure 2]]). In many fields, unconformities are the location of sealing shales and/or source rocks above reservoir pay. Subcrop maps, traces of productive zones, barriers, or marker horizons mapped on the unconformity surface are invaluable for planning well placement and for reservoir development.

 

===Pressure===

===Pressure===

Maps of reservoir pressure are useful throughout reservoir life (Figure 3). Pressures should be converted to a common depth datum, such as mid-reservoir, prior to contouring. (For information on obtaining pressure data, see the chapters on [[Production testing]] and [[Pressure transient testing]] in Part 9, “Wireline Formation Testing” in Part 4, and [[Drill stem testing]] in Part 3.)

Maps of reservoir pressure are useful throughout reservoir life ([[:file:subsurface-maps_fig3.png|Figure 3]]). Pressures should be converted to a common depth datum, such as mid-reservoir, prior to contouring. (For information on obtaining pressure data, see the chapters on [[Production testing]] and [[Pressure transient testing]] in Part 9, “Wireline Formation Testing” in Part 4, and [[Drill stem testing]] in Part 3.)

 

==Mapping thicknesses==

==Mapping thicknesses==