PETROLEUM PROSPECT AND RISK EVALUATION OF A PART OF CACHAR BASIN, ASSAM, INDIA

Abstract

Assam and Assam Arakan, a Tertiary petroliferous province occurs in the north eastern part of Indian subcontinent, which has traditionally been divided into Assam Shelf, Cachar Fold Belt and Tripura Fold Belt by the Oil industry. Assam Shelf, Tripura and the adjacent basins in Bangladesh have several oil/gas discoveries in Tertiary sandstones. However, there have not been many successes in Cachar Fold Belt although a few gas fields like Adamtila, Banaskandi have been discovered. Hence a part of Cachar Fold Belt has been studied in the realm of petroleum prospect evaluation and geologic risk analysis. A systematic approach has been made to quantify the geological risk. The geoscientific data include approximately 250 GLK of 2D seismic data, 54 sq km of 3D seismic data and data of 30 wells. Further geochemical analytical data based on 559 samples from seven wells and outcrops was used to evaluate source rock potential. Conclusions on stratigraphic information are based on analytical data of innumerable samples from wells and outcrops. A thick pile of more than 11 km of Tertiary sediments are known to occur in this Basin. Lithostratigraphically the post Paleocene sediments have been divided into four Groups from bottom to top viz. Barail (Late Eocene-Oligocene, comprising of Laisaong, Jenam and Renji Formations from bottom to top), Surma (Early Miocene to early Pliocene comprising of Bhuban and Bokabil Formations), Tipam (Pliocene) and ,Dupitila (Plio-Pleistocene) on the basis of surface geological investigations and well data. From a hydrocarbon exploration point of view, Bokabil and Tipam Sandstone Formations are the main plays. Petroleum Prospect Evaluation The oldest section drilled in Cachar Fold Belt is Oligocene Renji Formation. Source rock studies carried out on samples from wells have indicated that no effective source rock sequences have been encountered. Analysis of outcrop samples of the argillaceous Oligocene Jenam Formation established it to be effective source rock. As the drilled wells have not penetrated the source rock sequences, source rock maturation modelling was carried out to decipher the source-generation-migration history in the Nilambazar syncline, kitchen area. Parameters like VRo, Transformation ratio, bulk petroleum generated/expelled were assessed and the critical time was deduced to be around 7 to 5 Ma. The generation and expulsion is envisaged to be a continuing process through and beyond the Pliocene folding episode. It is concluded from all these exercises that Oligocene Jenam Formation is the effective source rocks which have adequate organic content, generation potential and undergone thermal maturity to have generated and expelled hydrocarbons. This corroborates with the source rock analysis carried out on outcrop samples and source rock correlation studies. Estimate of generated hydrocarbons An attempt has been made to estimate the bulk petroleum that would have been generated by the Jenam Formation source rock during the main generation period. A mean value of 351.7 MMTOE/2461 MMBOE of hydrocarbons are estimated to have been generated in the Adamtila area. As the stratigraphic succession in the synclines of Cachar Fold Belt is similar, the source rock modelling carried out can be considered to be representative of the entire synclinal area of Cachar Fold Belt. A mean value of 3199.8 MMTOE of hydrocarbons are estimated to have been generated and 288.75 MMTOE are estimated to have been trapped in reservoirs, which indicates that there is a huge Yet To Find Potential in the Basin. Identification of Prospect Areas Entrapment in Adamtila field is essentially structural and controlled by the N-S thrust which binds the field in the west. The thrust and the resultant structures provide entrapment situation. Seismo geological cross sections were prepared based on 3D seismic data tied with well logs. Mapping based on interpretation of the seismic data, well data and cross sections led to identification of two levels viz. Bokabil Formation and Tipam Sandstone Formation to be interesting in two prospective areas (i) Area I, East of the main Adamtila Field and (ii) Area II, North of the main Adamtila Field. Geological Risk Assessment Geological risk can be viewed as the most important risk factor in the business of petroleum exploration. The geological risks associated with the Prospect Areas were assessed in an objective manner. One of main objectives was to assess the geological risk quantitatively with minimum possible uncertainty. Quantitative assessment of Geological Risk Quantification Probability of success (POS) is essential to compute the Expected Monetary Value (EMV) of a prospect. It has been established that different geoscientists interpreting the same data set assign probabilities in a wide range. Considering these factors, the exploration team members were requested to assign the probability independently for the prospect areas based on their perception. The team members were associated with the entire process of interpretation of the seismic data, well data and source rock modelling exercises. The Probability assigned for (i) Bokabil Formation in Area I ranged from 0.154 to 0.3744 (ii) Tipam Sandstone Formation in Area I ranged from 0.11 to 0.33 (iii) Bokabil Formation in Area II ranged from 0.13 to 0.35 (iv) Tipam Formation in Area II ranged from 0.14 to 0.30 which indicates the differences in perception amongst team members. The values assigned by the members for the prospect areas were used to run Monte-Carlo simulation with a sensitivity analysis. The results of the sensitivity analysis in terms of contribution to variance were used to construct a radar diagram called "Uncertainty Web"\o pictorially depict the influence of the parameters on Probability of Success. This aids in immediate recognition of parameters whose assessment is beset with uncertainties. The Uncertainty Web only indicates the uncertainty in estimation of a particular parameter and not the relative riskiness amongst the parameters. After all these exercises, the parameters were revisited. A series of tasks were undertaken which included additional source rock modelling, attempts on sequence stratigraphy, amplitude analysis and relook into the mapping aspects in the seismic volume. The following conclusions were drawn after such a revisit, (i) Jenam Formation is the main effective source rock which has quality, quantity and also maturity to have expelled hydrocarbons, (ii) The entrapment model was checked again in the 3D volume. It was concluded that the integrity of mapping was fair enough to make decisions. (iii) The main target horizons in the area viz., sands within Bhuban, Bokabil and Tipam Formations are charged through the faults which have acted as conduits. (iv) The expulsion time superimposed on burial history clearly brings out that the expulsion from Jenam Formation started at around 14 Ma and is continuing. This leads to the conclusion that the structures which formed from Late Miocene to Pliocene would have trapped the expelled hydrocarbons. (v) However the uncertainty in reservoir continuity within Bokabil Formation remained. An attempt was made towards constructing Sequence stratigraphy models in the study area. This was however constrained due to lack of cores, paleontological data and amenable seismic data. Moreover, the results of the attempt are not presented here due to proprietary nature of the inferences. (vi) Another area where no substantial results could be obtained was in analysis of seismic amplitudes. The 3D seismic data acquired over the Adamtila anticline was the first ever campaign in Cachar Fold Belt. Though there was a vast improvement over the earlier 2D data, there was significant loss of amplitudes over the anticline which could not facilitate any meaningful amplitude based interpretation to characterise pay sands. With these conclusions differences in perception had considerably reduced and the Probability of Success was re-estimated as (i) 0.2426 for Bokabil Formation in Prospect Area I (ii) 0.1782 for Tipam Sandstone Formation in Prospect Area I (iii) 0.2252 for Bokabil Formation in Prospect Area II and (iv) 0.1620 for Tipam Sandstone Formation in Prospect Area II. Thus it can be concluded that creation of Uncertainty Web in order to reduce the uncertainties would be one of the best ways to address the issue of quantification of geological risk. This would also lead to a robust prospect appraisal methodology. Techno-economic Analysis A detailed techno-economic analysis was carried out to assess the economic viability of the prospect areas. However, these economic indicators do not incorporate the geological risk. Hence it is important to incorporate the geological risk and compute EMV. The different economic and risk indicators indicated that exploratory drilling in the prospect areas would be a viable venture. The EMV analysis also indicates that Prospect Area I can be prioritised over Prospect Area II. Exploration Portfolio Analysis Two prospect area with two target horizons each have been identified. The combination of resultant events would be four each in the prospect area I and II and sixteen when considered together. The probability assigned for each play/prospect area was used to assign probabilities at the portfolio level. The plays Bokabil Formation and Tipam Sandstone Formation in two prospect areas I and II however share commonalities in respect of source and play dynamics. The Jenam Formation source rock has fed both the prospect areas I and II and the play dynamics in terms of timing of migration are common for the play in both the areas, though the reservoir and entrapment situation are independent factors. In the case of Bokabil Formation the probability of success in both areas I and II was estimated to be 0.0867 and the probability of going dry in both the areas was estimated to be 0.6189. Similarly in the case of Tipam Sandstone Formation, probability of success in both areas I and II was estimated to be 0.0802 while the probability of going dry in both the areas was estimated to be 0.74. Based on these estimates of probability, the EMV for Bokabil Formation Play and Tipam Sandstone Formation Play were computed to be US $ 129.62 MM and US $ 32.15 MM respectively. IV Based on all these exercises, the Area I can be prioritised with Bokabil Formation as the primary play. The following are the salient features of the study 1. The source rock modelling indicated that the Oligocene Jenam Formation is the main effective source rock in the area. It appears to have generated about 3199.80 Million Tonnes of Oil Equivalent, MMTOE in the entire Cachar synclinal area and 351.70 MMTOE in the Nilambazar-Lalkhira synclinal area adjoining Adamtila. Hence, similar prospects adjacent to other anticlinal structures in the area shall be of interest as they would be proximal to source kitchens and this indicates that there is a vast Yet To Find Potential in Cachar Fold belt. 2. Two prospect areas were identified on the basis of detailed mapping with an ultimate reserves of the order of 10 Billion Cubic Meters of gas. 3. The geological risk related to the prospect areas was analysed and an attempt has been made to develop a methodology for Quantification of Geological Risk through a new concept of "Uncertainty Web". Quantification of geological risk has been carried out progressively moving from a qualitative approach taking cognisance of the perceptions of the team members, through extensive use of Monte Carlo simulation and a revisit to the geological aspects. The probabilities were assigned after such exercises of revisit, by reducing the uncertainty through the Uncertainty Web methodology. The EMVs were calculated for Area-I and Area-ll through decision tree exercises. Based on all these exercises, Prospect Area I was prioritised over Prospect Area II with Bokabil Formation as the primary play. 4. The utility of "Uncertainty Web" has been demonstrated in portfolio analysis as well. As part of portfolio analysis of the study area, the dependent and independent factors that control Probability of Success have been given due credence and the play wise probabilities have been estimated. 5. A Portfolio Risk Matrix has been devised incorporating technical and commercial parameters. The Portfolio Risk Matrix provides various options for prudent risk taking considering technical and commercial parameters and strategy and bias as well. A new paradigm called "Comfort" has been designed and included in Portfolio Analysis and prioritisation methodology.