Shallow Pays in Sirikit Main Oilfield, Phitsanulok Basin, Onshore Thailand: Their Habitat and Identification on 3D Seismic Data
Authors: Laoniyomthai Nuntanut , Charoenpun Tanaporn , Saifuddin Farid 
Published: May 2016

Shallow hydrocarbon pays were occasionally encountered above Main Seal, the top seal for Lan Krabu K reservoirs, uppermost main reservoir in the Sirikit Main Field. Shallow hydrocarbon is trapped in the Lower Yom and PTO Formations sandstone reservoirs which in the past were rarely considered to be drilling target in the field. Some accumulations were discovered unintentionally while drilling into the deeper Lan Krabu reservoirs. In the recent years, more effort is directed to the shallow pays as they often give considerable production contribution.

This paper will cover habitat and identification of shallow pays on seismic data. Their traps and seals were examined and compared to the main pays. The source rock, kitchen and migration are interpreted to be somewhat similar. The reservoir characters are of high quality braided and meandering river sandstones. Seismic reservoir characterization and hydrocarbon indication in the shallow reservoirs are feasible due to their acoustic properties.

Two types of trap are proven: structural and combined traps. Faulted anticline is the major structural trap. The combined trap is formed by confined channel complex, bounding faults and closing dip as in CFR-F block Upper PTO. Intra formational clays act as vertical seals while lateral seals are formed by faults. The vertical and lateral seal capacity is considered smaller compared to the thick Chumsaeng shale for the main Lan Krabu reservoirs.

Seismic anomalies correspond to the proven shallow pays. The D block PTO oil accumulation is associated with bright spot and notable flat spot corresponds to the oil water contact. Nong Pluang Yom and CFR-F block PTO gas accumulations are identified as low frequency bright spot on seismic data. Seismic attribute was successfully utilized for reservoir characterization and well targeting in the CFR-F block Upper PTO oil accumulation. A mapped Lower PTO seismic anomaly in the northern flank was successfully tested by LKU-U06 well with 23 m of oil pay. LKU-U06 initial production rate from PTO was 2000 BOPD, represented about 10% of overall field daily production rate. These encourage the working team to put more effort to find and develop more shallow pays in the field.

Introduction

The Sirikit Main Field is located in Phitsanulok Basin, Central Plain, onshore Thailand, approximately 400 km north of Bangkok (Figure 1). The Field is located in the S1 concession, operated and wholly owned by PTT Exploration and Production. The Field was discovered in 1981 by drilling LKU-A01 well.

Sirikit Main hydrocarbon accumulation is mainly found in the Miocene Lan Krabu formation K and L reservoir members with additional accumulation in the deeper and shallower reservoirs. Pre-Tertiary basement, Oligocene Sarabop - P formation and Lan Krabu M member are the deeper reservoirs. Lan Krabu K and L are estimated to hold more than 88% of Sirikit Main total hydrocarbon in place volume. Shallow hydrocarbon is trapped in the Lower Yom and Pratu Tao Formations sandstone reservoirs which in the past were rarely considered to be drilling target in the field. Pratu Tao reservoirs contribute around 3.2% of the field's in place hydrocarbon volume.