Method for interpreting seismic records to yield indications of gas/oil in an earth formation

System for resolving event characteristics of a seismic record so as to determine gas/oil fractions in a selected zone of an earth formation by creating synthetic gas-indicating characteristics from brine-saturated petrophysical parameters corresponding to each selected zone of interest, comprising a programmable digital computer including a CPU-control means, program and data memories connected to said CPU-control means and display-storage means also connected to said CPU-control means, said CPU-control means operating on petrophysical data within said data memory control of program statements initially held in said program memory but sequentially fetched to said CPU-control means so as:

• (i) from brine-saturated bulk and shear moduli (Kw* and Gw*), calculated by empirical equations including a P-wave modulus value (Pw*) which corresponds to said zone of interest based on well logging geologic data and estimated frame modulus values (Ka) of said zone of interest, to predict bulk modulus values Kg* with the gas/oil present in differing amounts in said zone of interest;
• (ii) to determine P-wave modulus values of the zone with gas/oil present (Pgas/oil*) in differing amounts based on the relationship
• P.sub.gas/oil *=K.sub.gas/oil *+(4/3)G.sub.w *
• (iii) to predict from seismic velocities within said zone (Pgas/oil*) calculated from step (B) as a function of gas/oil saturation;
• (iv) to compare said predicted seismic velocities or derivations thereof with actual velocities or similar derivatives thereof from said seismic record to predict amounts of gas/oil fractions in said zone of interest.

System for resolving event characteristics of a seismic record so as to determine gas/oil fractions in a selected zone of an earth formation by creating synthetic gas/oil-indicating characteristics from brine-saturated petrophysical parameters corresponding to each selected zone of interest, comprising:

• (A) a programmable digital computer for:
• (i) from calculated normalized brine-saturated petrophysical parameters involving a laboratory and/or well-log generated brine-saturated P-wave modulus value for a brine-saturated rock corresponding to said zone of interest, predicting compressional velocities of said zone with gas/oil in differing amounts present in said zone, in accordance with the relationship ##EQU16## where .rho.b is the bulk density, .phi. is the porosity, G* is the shear modulus at the saturations of interest, Kfg equals the bulk modulus for the liquid-gas mixture, Ka is the frame modulus, Km is the bulk modulus of a hypothetical rock, i.e., same composition but perfectly cemented with zero porosity;
• (ii) from seismic velocities of said zone calculated from (i), predicting synthetic seismic amplitude information with gas/oil present in said zone;
• and
• (iii) comparing said predicted amplitude information with gas/oil present with actual field-acquired amplitude data associated with said seismic record so as to determine both presence and amounts of gas/oil within said zone of interest.

Method for resolving event characteristics of an acoustic record so as to determine gas/oil fractions in a selected zone of an earth formation by creating synthetic gas-indicating characteristics from brine-saturated petrophysical parameters corresponding to each selected zone of interest, comprising:

• (a) predicting brine-saturated bulk and shear moduli (Kw* and Gw*) using empirical equations containing coefficients and a P-wave modulus value (Pw*), said coefficients varying in empirical fashion as a function of at least porosity, pressure and temperature, said P-wave modulus value (Pw*) being a P-wave modulus value which corresponds to said zone of interest based on well logging geologic data;
• (b) estimating a frame modulus value (Ka) of said zone of interest using a relationship involving Kw*, Kb, Km and .phi., where Kw* is brine-saturated bulk density of the aggregate, Kb is the bulk modulus of brine, and Km is the bulk modulus of the solid material making up the zone of interest and .phi. is porosity;
• (c) predicting a bulk modulus value Kg* with the gas/oil present in said zone of interest in a predetermined amount using a relationship involving Ka of step (b), km, .phi., Kfg, where Kfg is the bulk modulus of a gas/oil mixture;
• (d) assuming Gw* is equal to Ggas/oil*, predicting P-wave modulus value of the zone with gas/oil present (Pgas/oil*) in said predetermined amount based on the relationship
• P.sub.gas/oil *=K.sub.gas/oil *+(4/3)G.sub.w *
• (e) predicting at least an acoustic velocity within said zone from said P-wave modulus value (Pgas/oil*) of step (d);
• (f) repeating steps (c)-(e) for gas/oil-brine mixtures of different gas/oil saturations to provide a suite of synthetic acoustic velocities for said zone of interest as the function of a plurality of gas/oil saturations;
• and
• (g) comparing said suite of predicted velocities with actual velocity information produced from said actual acoustic record to determine presence and amounts of gas/oil fractions in said zone of interest.

Method for resolving event characteristics of an acoustic record so as to determine gas/oil fractions in a selected zone of an earth formation by creating synthetic gas/oil-indicating characteristics from brine-saturated petrophysical parameters corresponding to each selected zone of interest, comprising the steps of:

• (a) from calculated normalized brine-saturated petrophysical parameters involving a laboratory or otherwise annotated brine-saturated P-wave modulus value corresponding to said zone of interest, predicting P-wave modulus values with gas/oil in differing amounts present in said zone, in accordance with the relationship
• P.sub.gas/oil *=K.sub.gas/oil *+(4/3)G.sub.w *
• where Pgas/oil* is the predicted P-wave modulus with gas/oil present in predetermined amounts;
• Kgas/oil* is the predicted bulk modulus of said zone with gas/oil present in said predetermined amounts;
• Gw* is the brine-saturated shear modulus of said zone;
• (b) from acoustic velocities of said zone calculated from (a), predicting synthetic acoustic values associated with different gas/oil fractions present in said zone;
• and
• (c) comparing said synthetic acoustic values with actual field-acquired values so as to determine both presence and amounts of gas/oil within said zone of interest.

Method for resolving event characteristics of an acoustic record so as to determine gas/oil fractions in a selected zone of an earth formation by creating synthetic gas/oil indicating characteristics from brine-saturated petrophysical parameters corresponding to each selected zone of interest, comprising the steps of:

• (a) from calculated normalized brine-saturated petrophysical parameters involving laboratory and/or well-log generated brine-saturated P-wave modulus values corresponding to said zone of interest, predicting velocities of said zone with gas/oil in differing amounts present in said zone, in accordance with the relationship ##EQU18## where .rho.b is the bulk density, .phi. is the porosity, G* is the shear modulus at the saturations of interest, Ka is the frame modulus, Kfg equals the bulk modulus for the liquid-gas mixture, Km is the bulk modulus of a hypothetical rock, i.e., same composition but perfectly cemented with zero porosity;
• (b) from velocities calculated from (a), predicting synthetic acoustic information as a function of gas/oil present in said zone;
• and
• (c) comparing said predicted acoustic information with actual field-acquired data of said acoustic record so as to determine both presence and amounts of gas/oil within said zone of interest.

Method for resolving events of a seismic record associated with a selected zone of an earth formation so as to determine gas/oil fractions therein by creating synthetic gas/oil indicating characteristics from brine-saturated petrophysical parameters corresponding to said selected zone of interest, comprising the steps of:

• (a) from calculated normalized brine-saturated petrophysical parameters involving a laboratory or otherwise annotated brine-saturated P-wave modulus value corresponding to said zone of interest, predicting P-wave modulus values of said zone with gas/oil present in differing amounts in said zone in accordance with the relationship
• P.sub.gas/oil *=K.sub.gas/oil *+(4/3)G.sub.w *
• where Pgas/oil* is a predicted P-wave modulus with gas/oil present in said predetermined amounts;
• Kgas/oil* is a predicted bulk modulus of said zone with gas/oil present in said predetermined amounts;
• Gw* is the brine-saturated shear modulus of said zone;
• (b) from seismic velocities calculated from (a), predicting synthetic seismic amplitude values with gas/oil present in said zone in said predetermined amounts, and
• (c) comparing predicted amplitude values with actual values of said seismic record whereby amounts of gas/oil within said zone of interest can be accurately estimated.