SPE Thailand Annual E&P Award 2012

Company: PTT Exploration and Production PCL Project: Arthit Asset

Discovery of Deep Oligocene Syn-rift Lacustrine Reservoirs :

From Subsurface to Production

Project leader: WARANON LAPRABANG SPE 3265520

Project team members: SUPAMITTRA DANPANICH SPE 3578163

AWIRUT SIRIMONGKOLKITT SPE 3518533

KOMON PINYO SPE 3523125

KRONGPOL SEUSUTTHIYA SPE 4059712

THANAWUT WORAWUTTHICHANYAKUL SPE 3321006

Innovation Feature

Arthit project drilled a development platform in Q3 of 2011 at the Southern end of Tonson Fault Trend. The original main targets are fluvial dominated channel and bar sands of the post-rift reservoirs, which proved to be prospective and productive in the North Malay Basin. Referring to its locality, syn-rift packages can be imaged by available 3D seismic data. Map of top syn-rift sediment package, referred to H90 seismic marker, was mapped and were used in the conceptual well planning. TD of each well was set at approximately 150 meter below H100 seismic horizon, characterized by bands of strong amplitude anomaly.

After a few wells were drilled in the first batch, subsurface team revisited the geological model of the area and proposed to deepen well TD. The objective was to investigate the formerly known water bearing reservoirs, which structurally was drilled on an updip location. TD extension was success which discovered new reserves in the syn-rift lacustrine sequences below H100 seismic marker (Figure 1). Reservoir rocks are exhibiting good quality even at depth below 3000 mTVD-MSL. Multiple gas samples were taken from these newly discovered reservoirs which yield a moderate CO2 content.

Apart from the geological discovery of lacustrine syn-rift sections, this accomplishment was also proved successful by actual gas production information. After drilling operation finished in Q1 of 2012, wells were hooked up and initially perforated these newly discovered reservoirs. Initial production was started in April 2012 with 6 wells perforated in the reservoirs below H100 seismic marker. Even though the permeability estimated by an available poro-perm correlation indicates tight formation, actual production proved existing producibility of these syn-rift sands. Initial gas flow rate ranges from 3 to 13 MMscfd with various reservoir properties (10-21 % porosity). The produced gas contains moderate CO2 content ranging from 21-42 % with low condensate-gas ratio and gas heating value. The production from these syn-rift lacustrine reservoirs can be applied to the future nearby development wells in term of production-to-log calibration, well completion design, and etc. With the available production information, the new production technologies for improving recovery of these new pay zones are current in progress.

Figure 1. Hydrocarbon pay schematic; Proved New Reserves is identified below H100 seismic marker

Thai E&P Industry Impact and Value

Though syn-rift lacustrine sequences were discovered and produced before in Arthit and elsewhere in North May Basin but none were below H100 seismic marker. A local basement high located in the North Arthit area created an opportunity to drill into these syn-rift sections where impossible in other location due to its depth. Though structurally North Arthit is shallower than others, drilling into lacustrine syn-rift section is still a challenging task. The general depth of top syn-rift sediments are approximately 3000 mTVD-MSL with average geothermal gradient of 5.4°C/100 m. The success of the Oligocene syn-rift below H100 seismic marker marked the beginning of the appraisal in syn-rift sequences. The success impact much of the appraisal program which open new opportunity in certain areas where drilling into syn-rift is appropriate. The discovery also lead to new mapping of the strong amplitude anomaly sequence which revealed potential areas where structurally is interesting to appraise.

Two appraisal wells were drilled into Oligocene syn-rift sequence later in the year as a consequence of the discovery. Many additional zones of interests were encountered where high resistivity sections were encountered in syn-rift section with minor gas sample from fluid sampling program. Another deep well was drilled 500 meters below H100 seismic marker. Results were still doubtful due to lack of wireline information as a result of well control issue.

The impact of these newly discovered reservoirs below H100 seismic marker to the appraisal program is significant. Within Arthit project, many areas were revisited to relocate potential appraisal targets in the syn-rift section (Figure 2). The consequent two locations were drilled with a fair result while showing another kind of potential in Oligocene section. Future study will be focusing in understanding these potential zones.

Similar concept can be applied to other part of the basin and also to other basins in the Gulf of Thailand. Pre-Tertiary rift basin is interpreted as a lacustrine in origin which provides potential source rock varying in quality. Drilling into lacustrine syn-rift itself is a preferable case when reservoir rocks are encased by source bed of the lacustrine shales. Drilling in the Gulf of Thailand has been limited by drilling technology and the abundance of hydrocarbon in the post-rift sections.

Recent drilling technology allows the drilling into deep and hot environment. Most of the current main pay intervals are post-rift fluvial channel and bar sands which have been producing for decades. Many potential syn-rift plays are then the new hope to maintain the gas production in the gulf.

The monetary value of this discovery is countless, due to its vast application to other portion of the basin and to even other basins in the Gulf of Thailand. The least amount of additional monetary value of this discovery can be estimated from the first platform that actually produced from these reservoirs. Value of more than 180 MM THB was estimated to contribute from these Oligocene reservoirs below H100 seismic marker.

Figure 2. Random seismic line showing the Oligocene syn-rift section (green), noted the variety of potential plays which yet to be explored.

Why this project should win the award

The discovery of hydrocarbon sand below H100 seismic marker signifies the new exploration era for deep syn-rift rock unit. Subsurface team worked intensely with the drilling engineers to overcome the drilling operation limitation, which mainly contributed from high temperature nature of the area. One of the main contributors to this discovery was the extra work team spent on revisiting subsurface information and decided to extend TD.

The impact of this discovery was significant to other project where drilling into syn-rift section is appropriate. Neighboring projects also apply the similar concept to explore deeper into section where limited by past technology.

Good team work among subsurface and drilling engineers was achieved by hard work and dedication. This set up the good example to overcome challenges and barrier between disciplines. Similar model was then applied to other team to try accomplishing the same goal. Team was pushing many things to the limit which now becoming one of the best practice to share among the community.

Apart from the technological breakthrough, intellectual breakthrough was the main contributor to the success of this discovery. Team has to think outside the box to break mental barrier and bravely proposed to further appraise the section. Without this intellectual breakthrough, team would be doing routine work and lose the opportunity to find more hydrocarbon.