there is a possible VE-33 look alike east of the well as seen in Figure 8. The AVO anomaly particularly looks very good in this prospect. Figure 9, an impedance ...
Pinpointing new potential of mature fields in Mexico: An Example from Vernet field. by Sunit K. Addy*, J. D. C. Gutierrez**, Rafael J. de la Rosa**, F. Mosqueda**, Gwenaele Petit*, Mike Pelissier*, M artina Da Silva*, Miguel Garcia*, Sergio Ibañez*, Flavia Garbarino*, René Martinez* and Jean Louis Gelot*. *Compañía Mexicana de Geofísica, Villahermosa, Mexico, and **PEMEX, Cd. Pemex, Mexico. Summary An Integrated Study of the Vernet field in Mexico provided new information for the interpretation of deeper prospects. The initial success of drilling recommendations has opened the way for further exploration in this mature field. Introduction In 2002, Compañía Mexicana de Geofísica (CMG), conducted an integrated study of the Vernet field. Located in the southern part of the Macuspana basin and discovered in the 1930’s, this field has produced 4.4 mmb of 32-degree oil and 12 bcf of gas from 13 intervals in approximately 25 wells. The Macuspana basin is a mature basin in Southeast Mexico where hydrocarbon was discovered in the early part of the 20th century. It is a predominantly gas-prone basin with production of 5.4 TCF of gas and limited oil from Mio-Pliocene clastics in approximately 35 fields of varying sizes (Figure 1). Pemex estimates that there is still a substantial amount of undiscovered hydrocarbon in this 160 x 50 km basin. At the end of the study, several recommendations were made for additional exploration opportunities. The first proposed location, drilled by Pemex in December 2002 resulted in a success and will be presented here. Subsequently, Pemex has planned for additional drillings on this discovery to delineate the reservoir and to drill other recommended prospects. During the process of the integrated study and in the aftermath of the discovery, a 3D stratigraphic inversion and an Amplitude vs. offset (AVO) analysis were performed and the results will also be presented.
Figure 1: Location map of Macuspana basin and Vernet field.
NW
SE
Z4 Z5 Z9 Z10 E15
Main Fault
AS50 AS60
Geology Figure 2 is a cross line through the field showing our interpreted horizons. It is an extremely complex area consisting of a major growth fault and numerous subsidiary synthetic and antithetic faults creating small traps at various intervals. Production comes from the crestal fault-bounded traps located on the downthrown side of the major fault. Most of the production comes from the Z9 to E15 interval on the structure. Production from AS 50 and deeper sands is very limited (40000 bbls and 1 mmcf) and is recorded only in two wells. In general, it was believed that the sand quality deteriorates with depth which is no longer true. The depositional facies varied from non-marine, channel, near shore to offshore.
Figure 2: Line showing the complex Vernet anticline. The synthetic ad antithetic faults have created numerous traps at various intervals. Most production comes from Z9 to E15 intervals.
Value added processing The 3D data set shot by COMESA in 1997-98. The data quality was poor to good. There were large skips and numerous faults in the Vernet field area. The data was further processed for noise reduction, stratigraphic inversion, pre-stack time migration and AVO. The post stack original seismic was improved by spectral whitening and FXY de-noising program in Geovecteur to boost the amplitude contained in the high frequency.
Pinpointing new potential of mature fields in Mexico A TDROV (Three D Rho-Velocity) stratigraphic inversion on the post-stack time-migrated seismic cube was performed. It is a 3D post-stack stratigraphic inversion program which uses a global optimization technique (simulated annealing) to iteratively build a stable highresolution layer framework of acoustic impedance. To prepare the seismic data for inversion, well logs and synthetic seismograms were used 1) to determine the phase of the data to make it zero phase, 2) to generate a stratigraphic deconvolution operator to match amplitude spectrum of the seismic data with the synthetic data, 3) to derive a regional wavelet from the seismic data, and 4) to estimate impedance values at the top of the mapped horizons and corridors of possible impedance between horizons. For wells without density logs, Gardner’s relationship for converting sonic to density was used. A simple initial model was then constructed by assigning impedance values to the horizon maps (macro layers) which were generated at key levels encompassing the zones of interest to be inverted. Between these macro layers the program defined the micro layers whose thickness was determined by the width of the regional wavelet. The purpose of micro layers was to create time layered impedance grids used to simulate seismic reflectivity during the inversion process. The inversion process randomly perturbed both the time (at 1 ms intervals) and impedance along micro layers until the difference between the modeled traces and seismic traces stabilized. Because impedance does not vary linearly as a function of seismic amplitudes, simulated annealing was used which prevented the inversion solution from being trapped in a local minima. The inversion was performed on a 3D volume as opposed to a trace by trace inversion. After inversion, each micro-layer with its two way time, impedance and thickness variation was extracted and mapped automatically. Also a 3D impedance volume at a sampling rate of 1 ms was obtained for loading into the work station and for further analysis. Well logs are not directly input into the inversion program. This is especially critical since wire line logs and seismic data are two different kinds of measurements of the same rock properties. Therefore, the well information can be used to cross validate the seismic inversion and vice versa when the well data are in question. The TDROV inversion results in a higher resolution and a more laterally coherent data set than the original seismic data. Also the inverted data set is free from tuning effects and shows reduced noise. The impedance layers may be used in reservoir property mapping. For a detailed discussion of this procedure, please refer to Gluck et al., (1997). The interpretation of the inversion results is very encouraging. The presence of gas generates a distinct seismic response with a decrease in acoustic impedance if the layer is sufficiently thick.
The AVO analysis was done using Geovecteur program. Prior to AVO analysis, the seismic data were zero phased using well logs. The SEG positive polarity convention was used in which an increase in acoustic impedance (+ve Reflection Coefficient) is represented by a peak on a variable wiggle display. First synthetic 1D AVO modeling was done. Sonic, density and shear sonic are input into a modeling program (Easy Trace), which calculates a synthetic CDP gather using the same offset range and number of offsets as the seismic CDP gather. After the synthetic CDP gather was generated it was compared to the AVO response of our seismic data. Within a CDP gather the seismic reflectors were aligned as best as possible and because of that a detailed velocity analysis was done in the AVO study. At every CDP location at every sample rate an intercept (I) and gradient (g) were calculated. These values were used to generate two different AVO attributes, the product stack or I*G and the fluid factor plot. The I*G attribute of a low acoustic impedance reservoirs has negative intercepts and negative gradients at the top of the reservoir and positive intercepts and gradients at the base of the reservoir. In both the cases we get a positive number. The product stack (I*G) values are shown superimposed on the wiggle traces with positive product as red and negative product as blue. Areas of red suggest possible gas bearing sand. The fluid factor is a cross plot between the intercept and the gradient. The top of a hydrocarbon saturated low acoustic impedance reservoir is represented by a negative (blue) fluid factor anomaly, the base of such a sand by a positive (red) fluid factor anomaly. VE-33 Example As part of a push towards exploring deeper targets, which may provide additional exploration potential, it was recommended that an existing well, Vernet 33, should be extended to test a seismic anomaly (Figure 3). This anomaly was observed during the course of the study along with several other un-drilled deep and shallow prospects.
The prospect had a strong amplitude anomaly, a distinct low acoustic impedance and a good positive AVO anomaly (Figures 3, 5, 6, 7 & 8). Vernet 33 was drilled in 1957 and has been a producer in Z5, Z9, Z10 and Z13 with a cumulative total of 0.42 mmbo and 1.3 bcf of gas. In 2002 the well produced only 33 bpd from the interval 612-616 m (Z5) with the help of a pneumatic pump but the well was clean and was ready for a re-entry. The deep prospect below Vernet 33 is on a structural high at 1.4s , just below the AS 50 marker with a strong amplitude anomaly and low acoustic impedance (Figure 3). Figure 4 shows the results of the drilling. Vernet 33 at 1697 m encountered AS 51 sand (45 m gross and 40 m net)
Pinpointing new potential of mature fields in Mexico belonging to the Amate Superior Formation of Middle Pliocene age. The interval from 1715 to 1727 m was tested and it is presently producing 5.34 mmcfgpd from a sand with 31 % porosity, 622 md permeability and 15 % water saturation. Sands of this high quality have been found for the first time in deep Vernet and this has opened a new avenue for further exploration in this mature field.
with a positive intercept*gradient anomaly. Figure 8 is a wiggle trace line through VE-33 with clear red I*G anomaly at the well.
VE-33 SE
NW Crossline 1748
B4 Z4 B4 Z9 Z4 Z 10
Z9
* * *
Amplitude Anomaly Figure 5A 0
Z 10 E15 AS50 Bloque Productor
100 200 300 400 500 m
Line shown in figure 3
AS50 AS51
0
0,5
1 km
AS60
AS60
Crossline 1748
Figure 3. Cross line through Vernet 33 well. The well was drilled in 1957 to total depth of 1143 m in E15 at approximztely 1.15 ms. The deep prospect is at 1.4 s and shows a significant amplitude anomaly. This anomaly was investigated by inversion and AVO and a recommendation to deepen the V33 well was made. The figure shows the new V33 extension ( drilled in 2002) which encountered excellent gas sand. Impedance and sonic logs are shown in track 1 and track 2.
Figure 5B 0
100 200 300 400 500 m
Figure 5A and 5B. Time structure and amplitude anomaly maps of AS51, gold horizon shown in Figure 3. The anomaly does not follow the structure contours suggest that the sand is possibly variable in thickness and in quality. Pemex is presently developing this reservoir using impedance and AVO attributes.
Other deep anomalies
AS51
*
5.34 mmcfgpd
Figure 4. Vernet 33 deep logs showing the AS 51 sand with a net thickness of 40 m, 31 % porosity and 15 % water saturation. The flow rate in the perforated interval is 5.34 mmcfgpd
Subsequent to this discovery, additional localized inversion (Figure 7) and AVO were done in order to define the AS51 sand for reserve calculation and re-evaluate other deep prospects in the region. The modeling and the seismic response near the well compare well within reasons. (Figure 7). The response suggest that AS51 sand is a type III sand showing lower velocity and density in the gas bearing sands and a strong increase in amplitude vs. offset
The integrated study coupled with inversion and AVO has revealed several other deep and shallow prospects which are presently being considered for drilling. For example, there is a possible VE-33 look alike east of the well as seen in Figure 8. The AVO anomaly particularly looks very good in this prospect. Figure 9, an impedance in line clearly shows that all the three low impedance anomalies have been missed by the well.
Pinpointing new potential of mature fields in Mexico
Vernet 33
AVO Anomaly V33 look alike AS 51 SAND
AS 51 gas pool
Figure 8. Inline wiggle trace data through Vernet 33 with AVO Intercept*Gradient attribute superimposed. Red indicates gas as seen at the well. The synthetic shows an excellent match. A strong positive anomaly is observed to the right which has been classified as a Vernet 33 look alike. WELL
Figure 6A and 6B. Stratigraphic acoustic inversion cross line through the deep prospect and time slice through 1410 ms showing the AS51 gas pool. Such maps have been used for reserve calculation.
Figure 9. Other Vernet 33 looks alike. Inversion shows that the anomalies have been missed by the nearby well.
Conclusions The positive outcome of this integrated study shows that careful analysis of mature basins with value-added processing often yields hidden treasures such as VE-33 and perhaps many more like it. References Gluck, S., E. Juve and Y. Lafet, 1997, High resolution impedance layering through 3-D stratigraphic inversion of post-stack seismic data, The Leading Edge, Vol.16, pp.
1309-1315. . Figure 7A. NMO corrected gathers at V33 location at AS51 level showing AVO effect compared to model data from the well. The response is that of a typical Type III sand. The far traces in the gathers are absent due to skips or they have been muted out 7B. Gathers by angle showing the AVO effect at AS61 sand highlighted.
Acknowledgements We acknowledge help from various colleagues in Pemex and particularly, Ing. Molina and Ing. Flores for granting us permission to present this paper. We are thankful to Ms. S. Pink-Zerlink who edited an earlier version of this paper and Mr. J. Bencomo for computer graphics.
