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DIAGENETIC RESERVOIRS   227

               coast of South Andros Island in the Bahamas. The shape and extent of many cave
               systems is dictated by preexisting fracture systems, and this association may be
               especially useful for exploring and developing paleocave reservoirs. The caves of
               northeast Yucat á n, South Andros, and in the Guadalupe Mountains are related to
               fracture systems. It is certainly worth considering that paleocave reservoirs and
               aquifers may follow preexisting fracture patterns.
                   Two field examples of hybrid reservoirs with diagenetic attributes dominant were

               selected for this chapter. One of them illustrates diagenetic microporosity (neomor-
               phic stabilization diagenesis) in a grainstone reservoir at Overton Field, Texas. The
               other is an example of pore enlargement, new pore formation, and moldic porosity
               in a Permian grainstone and reef - rubble buildup at Happy Field, Garza County,
               Texas. These examples were chosen because they represent different styles of dia-
               genesis and pore types.

                   8.4.2.1   Overton Field


                  Location and General Background   Overton Field is one of the largest gas fi elds

               in the East Texas basin (Figure  8.10 ). It produces from altered oolite grainstones in
               the Jurassic Cotton Valley Limestone. Diagenesis affected only the oolitic facies on
               tops of paleostructural highs while relatively unaltered, nonproductive oolite grain-
               stones are present lower (older) in the stratigraphic section. The relatively unaltered
               oolite facies are also missing on paleostructural highs in the producing fairway.



                                                         OKLA

                                         N  0     10   20  TEXAS
                                                  mi

                                      MEXA-TALCO FAULT ZONE  ?  ?  ?  GRAINSTONES  Kilgore  ARK
                                                          OOLITIC
                                                                      LA



                                           ?  ?             ANCESTRAL
                                                              SABINE
                                                              UPLIFT
                                                                MUD-RICH  LS.
                                                       OVERTON





                                        McSwane
                                                 OUTER RAMP
                                                 CARBONATES
                                      Bear Grass
                                    Bald Prairie

                    Figure 8.10   Location of Overton Cotton Valley (Jurassic) gas field in East Texas. Note that



               the field is on the edge of an ancient structural feature known as the Ancestral Sabine Uplift.

               To the west, off the edge of the ancient uplift, is the East Texas salt basin where salt tectonics
               has created uplifts, domes, diapirs, and withdrawal basins.
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