Carbonate Basement Fracture Identification from Conventional Data: Well log, Drilling Parameter, and Petrography- A study case from “X-1” well, North Palembang Sub-Basin, South Sumatra, Indonesia

Abstrak

Most of the times, recognition of fracture evidences requires bundles of advance well data set that are costly and sometimes are not operationally practical to collect. This paper describes an approach to identify fractures evidence through more common data acquisition in a drilling. The data subject to wireline data, drilling data (drilling parameter and mudlog data) and petrography analyses from the cutting samples.

The “X-1” well was drilled with main objectives in shallower sandstone reservoir of Telisa Fm and secondary objectives in Talang Akar Fm. This well penetrated 75 m of carbonate basement as tertiary objective. Previous operator has not really explored the carbonate basement potential after the drilling completed as they regarded the interval as ‘tight’ and ‘non pay’ zone. Recent discovery of giant gas fractured reservoir in Sakakemang Block, around 100 km to the northwest of the X-1 well has triggered the current operator to research the potentiality of basement reservoir in the study area.

The carbonate basement interval in “X” structure was characterized by an erratic and spiky resistivity and porosity log (density, neutron, and sonic log) pattern. The lithology is mainly crystalline carbonate with average bulk density close to 2.7 gr/cc and sonic log reading 40-60 ms/ft indicating a very small amount of porosity.

Even so, some anomalies are recorded by the porosity logs in this interval when sonic porosity is less than density neutron porosity. These facts indicate that the secondary porosity was developed in the rock matrix. From the petrophysical evaluation, it can be estimated that the secondary porosity is ranging from 2% to 19%. As what encountered in the logs, secondary porosity in the dominantly tight basement interval is also demonstrated from the drilling break event. Where secondary porosity exists, one can notice an increase of ROP (rate of penetration) from average 19-20 minutes/meters to 5-8 minutes/ meters. Hydrocarbon indication emerges from notable increase in Tgas, C1 (methane) and C2 (ethane) on basement interval. Another indication comes from a well control situation when penetrating the basement interval with secondary porosity. During pulling out BHA after penetrating top of basement, the well was flowing and the flow was flared. To cope with the flow from the borehole, the mud weight was increased from average 1.08 ppg to 1.1 ppg.

Beside the well and drilling data, a petrography analysis from cutting samples in the interval with secondary porosity was carried out to figure out the existence of secondary porosity and what exactly the type of secondary porosity is in the rock matrix. The petrography analysis is showing 98% of type III/IV twinning-calcite, indicating a hot paleotemperature, with   intensive microfracture.  The fractures were mainly open with partial micaceous mineral and quartz overgrowth filling.  From the study, it is shown that the usage of common primary data such as wireline log, drilling parameter, and even cutting petrographic analysis, can be optimalized to investigate natural fracture occurrence in the basement.