No abstract

The major reservoir within the North Madura Platform is the Oligo-Miocene Interval, which consists of oil and gas-bearing reservoirs, with multiple reservoir layers and multiple hydrocarbon contacts. The interval comprises of a stacked high frequency cycles of mixed carbonate-siliciclastic lithology component sitting in the paleo-high. It has become very important in the context of hydrocarbon accumulation since the components play very dissimilar roles and become reservoir-seal pairs, which resulted in the existence of intraformational seal. Based on the carbonate to siliciclastic ratio, from deeper to shallower interval, it can be divided into three cycles, namely first cycle with carbonate-dominated interval, second cycle with mixed carbonate-shale interval, and third cycle consists of massive carbonate sequences. The vertical hydrocarbon migration, both oil and gas, occurred within the known trap in the study area, filled the reservoirs from deeper to shallower interval with subject to seal capacity of each intraformational seal and leak points in the trap. The first cycle consisted of oil indicated that the seal breached as reservoir become fully saturated and some of oil and most of the gas leak. The second cycle, which consisted of multiple reservoir-seal pairs with oil and gas being accumulated depending on the seal strength. The third cycle, which is massive carbonate reservoir, is capped by thick marine shale as a roof seal that has been proven for gas being trapped. Better understanding on intrafomational seal, both micro and macro characteristics, can be highly beneficial to identify potential overlooked zone as well as contributing to the proper judgment on the likelihood of hydrocarbon accumulation.

No abstract

No abstract

ABC-1 exploration well was drilled through a carbonate build-up structure of Ngimbang Formation in offshore East Java, Indonesia. Standard triple-combo open hole logs were acquired by means of logging while drilling, while more advance wireline loggings were planned subsequently. Unfortunately, there were total losses during drilling which had to be managed by pressurized mud cap drilling (PMCD) which prevent from cuttings recovery for the rest of the interval. Multiple trips were also required to drill the well safely resulting in rugose and enlarged borehole. These conditions did not allow open-hole wireline log to access the target located at the lower interval of the well. It was inevitable to complete the well despite not enough data had been acquired to perform a comprehensive formation evaluation. In order to obtain remaining required data, it was decided to complement the compromised open-hole data with an advance pulsed neutron log (PNL) device, which offered several unique measurements to tackle the harsh conditions. Some of these key measurements are: (1) a self-compensation algorithm which provided robust sigma (SIGM) and cased-hole porosity measurement (TPHI), which was used to further validate neutron from LWD. (2) A combination of both capture and inelastic high definition elemental spectra measurement were utilized to obtain accurate mineralogy fraction. (3) carbon-oxygen ratio (COR) high precision measurement to calculate oil saturation. Lastly (4), fast neutron capture cross-section (FNXS) measurement was also acquired to give insight on possible gas occurrence even in tight zones. The advance PNL, acquired over 3 passes, showed consistent reading of sigma, TPHI, FNXS and elemental spectroscopy measurement. However, there were some discrepancies in between COR passes, which eventually has shed some light on what happened in this well. The first pass did not really show any potential oil along the carbonate body. Then, the second pass started to reveal potential oil around the top part of the carbonate, where resistivity is low with no distinctive neutron-density crossover. The third pass revealed an even more oil volume along the top carbonate. There is a possibility that the increase of oil reading might be due to the changing environment during logging, allowing some invasion to dissipate along the carbonate tops. This implies that there might be yet another oil zone below the revealed oil interval, should the invasion fluid start to dissipate. Subsequent well test showed significant oil production over the interval identified from the PNL interpretation, which put ABC-1 as one of the most successful Indonesian exploration well in 2021. This case study shows the success of utilizing advance pulsed neutron log to perform comprehensive formation evaluation under challenging condition, which can be used as reference for tackling similar drilling challenges in the future.