Phitsanulok Gr

Phitsanulok Basin (northern Central Plain). The north-south trending Phitsanulok Basin is the largest individual rift basin complex in Thailand. Upward succession of the Sarabop Fm, Nong Bua Fm, Khom Fm, Lan Krabu Fm, Chum Saeng Fm, Prato Tao Fm, Tom Fm and Ping Fm. Largest oil field in Thailand.

Synonym: กลุ่มหินพิษณุโลก

[Figure: Map showing the Tertiary basins of Thailand, with Phetchabun basin highlighted. The solid black lines onshore are the principal rivers draining Northern and Central Thailand and the Khorat Plateau (Morley and Racey, 2011, page 224).]

Sirikit oilfield stratigraphy is considered typical: "The earliest stages of the rift, in the Late Oligocene – Early Miocene, were dominated by three facies: coarse, immature clastics deposited as alluvial fans (Sarabop Fm); alluvial plain deposits (Khom Fm); and fluvio- deltaic to lacustrine deposits (Nong Bua Formation). During the Early Miocene, deposition was dominated by lacustrine (Chum Saeng Fm) and fluvio-deltaic conditions (Lan Krabu Fm). The alternating environments juxtaposed fine-grained lacustrine shales, which are the oilfield’s source-rock and seal, against fluvio-deltaic sandstones which constitute the oil reservoir rocks. The main lacustrine episode occurred after deposition of the Lan Krabu Fm and forms the Main Seal in the oilfield. The Main Seal, as the name suggests, is the key top seal and lateral seal to hydrocarbon accumulations; it is part of the Chum Saeng Fm. Thinner lacustrine shales in the Lan Krabu Fm act as intra-formational seals, particularly the Lower Intermediate Seal (LIS) and the Upper Intermediate Seal (UIS) which separate different reservoir zone." (Morley & Racey, 2011). The sedimentary fill is locally up to 8 km thick, in contrast to the other rift basins in Northern Thailand where the deepest (the Fang and Chiang Mai basins) contain a maximum of 2 – 3 km of section.

Vertebrate fossils.

"It developed from the Late Oligocene to the Late Miocene or possibly Pliocene and exhibits marked facies variations, major shifts in the location of depocenters with time, multiple episodes of basin inversion that affect sub-basins in different ways, and several locally developed but important angular unconformities." (Morley & Racey, 2011)