Subsurface Maps K w. Weissenburger Conoco, Ine Ponca City, Oklahoma, U.S.A INTRODUCTION fields, unconformities are the location of sealing shales and/or source rocks above reservoir pay. Subcrop maps, Reservoir properties are mapped to promote optimal field traces of productive zones, barriers, or marker horizons development. Subsurface maps dictate well placement and mapped on the unconformity surface are invaluable for nable engineers to calculate reserves and monitor trends in Placement and for reservoir d reservoir performance. Geologists play a key role in subsurface mapping by using interpretations of depositional Pressure environments and diagenetic events to project reservoir data away from relatively few well control points(see other Maps of reservoir pressure are useful throughout reservoir chapters in Part 6). In this sense, subsurface mapping is in life(Figure 3). Pressures should be converted to a common reat contrast to geological mapping of the earth's surface. depth datum, such as mid-reservoir, prior to contouring.(For Whether using traditional concepts(Landes, 1951)or"high information on obtaining pressure data, see the chapters on technology"computer contouring hardware/software "Production Testing" and"Pressure Transient Testing"in systems (ones et al, 1986), mapping interwell areas places a Part 9, " Wireline Formation Testing "in Part 4, and"Drill premium on interpretation rather than straightforward Stem Testing"in Part 3. plotting of precise data. "Mapping"is here limited to ons MAPPING THICKNESSES MAPPING SURFACES Interpretations of depositional trends, pre- and yndepositional structural development, and reservoir A number of surfaces are typically mapped during storage capacity are based in large part on thickness reservoir development to show closure and other limits to information. An accurate meaning of thickness is critical in production. Maps of top of pay and bottom of pay these and other analyses (see the chapter on"Conversion of can also be"subtracted"to determine pay thickness Well Log Data to Subsurface Stratigraphic and Structural Information"in Part 6). Structure Structure maps show lines of equal elevation or depth for a selected marker horizon( Figure 1)(see the chapter on A contour map of equal values of true stratigraphic Evaluating Structurally Complex Reservoirs"in Part 6). thickness is an isopach map(Figure 4), Except for vertical wells Mean sea level is a useful reference datum. commonly in horizontal beds, corrections for wellbore deviation and contoured horizons are top of zone or top of net pay. Control formation dip are needed to make isopach maps oints are provided by surveyed wells and can be supplemented by seismic interpretations, especially offshore. Isochore In highly developed fields, typically onshore, sufficient well control might exist to allow geostatistical interpolation A contour map of equal values of true vertical thickness is between control points(see Part 8 n isochore map(Tucker 1988). Note that in common practice, isochore maps are informally referred to as "isopach"maps, a Fault planes term that properly should be restricted to true stratigraphic thickness Faults are special surfaces whose traces will show structure contour maps(Figures 1 and 2). Faults form Isochron bounding surfaces for some reservoirs, and sufficient well ontrol might exist to contour map the fault surface itself An isochron map is a contour map of equal values of seismic Projections of subsurface data into the plane of the fault are traveltime between selected events(Tucker,1988).Isochron appropriately described as cross sections. (For details of are intended to derive thickness information from seismic of fault pl e the ch data. Isochroning between events above and below a pay Conversion of Well Log Data to Subsurface Stratigraphic and horizon, for example, would estimate pay thickness. Renick Structural Information"in Part 6) and Gunn(1989)present a good case history of using isochron and time-structure maps to generate"isopach"and Unconformities and Subcrop levation-structure maps. Their isochron-isopach approach delineated reef trends for further development drilling and Surfaces of unconformity can be especially useful marker used well penetrations through a shallow horizon for depth horizons for structure contour mapping (Figure 2). In many control on a deeper horizon. Phipps(1989)documents theSubsurface Maps INTRODUCTION Reservoir properties are mapped to promote optimal field development. Subsurface maps dictate well placement and enable engineers to calculate reserves and monitor trends in reservoir performance. Geologists play a key role in subsurface mapping by using interpretations of depositional environments and diagenetic events to project reservoir data away from relatively few well control points (see other chapters in Part 6). In this sense, subsurface mapping is in great contrast to geological mapping of the earth's surface. Whether using traditional concepts (Landes, 1951) or "high technology" computer contouring hardware/software systems (Jones et al., 1986), mapping interwell areas places a premium on interpretation rather than straightforward plotting of precise data. "Mapping" is here limited to projections in plan view. MAPPING SURFACES 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 maps show lines of equal elevation or depth for a selected marker horizon (Figure 1) (see the chapter on "Evaluating Structurally Complex Reservoirs" in Part 6). 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 interpretations, especially offshore. In highly developed fields, typically onshore, sufficient well control might exist to allow geostatistical interpolation between control points (see Part 8). 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.) Unconformities and Subcrops Surfaces of unconformity can be especially useful marker horizons for structure contour mapping (Figure 2). In many K. W. Weissenburger Conoco, Inc. Ponca City, Oklahoma, U.S.A. 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 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.) MAPPING THICKNESSES Interpretations of depositional trends, pre- and syndepositional structural development, and reservoir storage capacity are based in large part on thickness information. An accurate meaning of thickness is critical in these and other analyses (see the chapter on "Conversion of Well Log Data to Subsurface Stratigraphic and Structural Information" in Part 6). Isopach A contour map of equal values of true stratigraphic thickness is an isopach map (Figure 4). Except for vertical wells in horizontal beds, corrections for wellbore deviation and formation dip are needed to make isopach maps. Isochore A contour map of equal values of true vertical thickness is an isochore map (Tucker, 1988). Note that in common practice, isochore maps are informally referred to as "isopach" maps, a term that properly should be restricted to true stratigraphic thickness. Isochron An isochron map is a contour map of equal values of seismic travelfime between selected events (Tucker, 1988). Isochron maps are the seismic analog of isochore maps and, as such, are intended to derive thickness information from seismic data. Isochroning between events above and below a pay horizon, for example, would estimate pay thickness. Renick and Gunn (1989) present a good case history of using isochron and time-structure maps to generate "isopach" and elevation-structure maps. Their isochron-isopach approach delineated reef trends for further development drilling and used well penetrations through a shallow horizon for depth control on a deeper horizon. Phipps (1989) documents the