US 4,375,090 A · Method for interpreting seismic records to yield indications of gas/oil in an earth formation such as a sandstone, limestone, or dolostone

Abstract

The present invention indicates that acoustic characteristics associated with gas/oil-containing strata such as a sandstone, limestone or dolostone, including reflectivity coefficients can be normalized (and favorably compared) with similar characteristics calculated and displayed by means of a machine-implemented data processing method in which well logging and geologic data are fed thereto to calculate such characteristics without the need for shear-wave velocities. In more detail, in accordance with the invention brine-saturated bulk and shear moduli, (i.e., Kw* and Gw*) of a sandstone, limestone or dolostone can be predicted as a function of, say, brine-saturated P-wave modulus (Pw*) alone (independent of shear-wave velocity). In that way, resulting acoustic values including seismic velocities and amplitudes (also, reflectivities) as a function of a saturation operator can ultimately be provided. Such values, when compared to actual field-generated characteristics, are surprisingly accurate predictors of the amount of gas/oil saturation in the zone of interest. The method has particular accuracy in designating gas zones within formations of interest due to use of the complete Benedict-Webb-Rubin non-ideal gas law in pseudo-reduced form, as cited in Journal of Current Petroleum Technology, vol. 22, pp. 889-895.

Claims10

Dependent claims

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, and comparing such characteristics with said record comprising:

(A) Means for obtaining said seismic record for at least said selected zone;
(B) Means for determining actual compressional velocity and density values of said zone of interest based on field information associated with a known zonal lithology selected from a group comprising sandstone, limestone and dolostone;
(C) A programmable digital computer for:
(i) from the parameters of (B), generating a brine-saturated P-wave modulus value corresponding to said zone of interest;
(ii) from said generated brine-saturated P-wave modulus value of (i), predicting compressional velocities of said zone with gas/oil in differing amounts present in said zone, in accordance with the relationship ##EQU12## where rho-b is the bulk density, .phi. is the porosity, G* is the shear modulus at the saturations of interest, Kf 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;
(iii) from seismic velocities of (B), predicting synthetic seismic amplitude information with gas/oil present in said zone in differing amounts;
and
(iv) comparing said predicted amplitude information with actual field-acquired amplitude data associated with said seismic record of (A) so as to determine both presence and amounts of gas/oil within said zone of interest.

5 more elements

Method 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, and comparing such characteristics with said record comprising the steps of:

(a) obtaining said seismic record for at least said selected zone by field measurement;
(b) determining actual compressional velocity and density values of said zone of interest based on field information associated with a known zonal lithology selected from a group comprising sandstone, limestone and dolostone;
(c) from the parameters of (b), generating a brine-saturated P-wave modulus value corresponding to said zone of interest;
(d) from said brine-saturated P-wave modulus value of (c), predicting P-wave moduli of said zone with gas/oil in differing amounts present in said zone, in accordance with the relationship ##EQU14## 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;
(e) from seismic velocities of said zone calculated from step (d), predicting synthetic seismic values associated with different gas/oil fractions present in said zone;
and
(f) comparing said synthetic seismic values with actual field-acquired values associated with said seismic record of (a) so as to determine both presence and amounts of gas/oil within said zone of interest.

6 more elements

(a) obtaining said seismic record for at least said selected zone by field measurement;
(b) determining actual compressional velocity and density values of said zone of interest based on field information associated with a known zonal lithology selected from a group comprising sandstone, limestone and dolostone;
(c) from brine-saturated P-wave modulus values corresponding to said zone of interest generated using the values of (b), predicting velocities of said zone as a function of gas/oil in differing amounts 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, Ka is the frame modulus, Kf 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;
(d) from velocities of said zone calculated from step (c), predicting synthetic amplitude information with gas/oil present in said zone;
and
(e) comparing said predicted amplitude information with gas/oil present with actual field-acquired amplitude data associated with said seismic record of (a) so as to determine both presence and amounts of gas/oil within said zone of interest.

3 more elements

Dependencies are inferred from the claim text.

Method for interpreting seismic records to yield indications of gas/oil in an earth formation such as a sandstone, limestone, or dolostone

Life of this patent

Method for interpreting seismic records to yield indications of gas/oil in an earth formation such as a sandstone, limestone, or dolostone

Life of this patent

Abstract

Claims10