Passage: By the 15th century, the empire became a Muslim state, when the King of Brunei converted to Islam, brought by Muslim Indians and Arab merchants from other parts of Maritime Southeast Asia, who came to trade and spread Islam. It controlled most of northern Borneo, and it became an important hub for the East and Western world trading system. Local historians assume that the Bruneian empire was a thalassocratic empire that was based upon maritime power, which means its influence was confined to coastal towns, ports and river estuarines, and seldom penetrated deep into the interior of the island. The Bruneian kings seem to have cultivated alliance with regional seafaring peoples of Orang Laut and Bajau that formed their naval armada. The Dayaks, native tribes of interior Borneo however, were not under their control, as empiral influence seldom penetrated deep into the jungles.Following the presence of Portuguese after the fall of Malacca, Portuguese merchants traded regularly with Brunei from 1530 and described the capital of Brunei as surrounded by a stone wall.During the rule of Bolkiah, the fifth Sultan, the empire held control over coastal areas of northwest Borneo (present-day Brunei, Sarawak and Sabah) and reached Seludong (present-day Manila), Sulu Archipelago including parts of the island of Mindanao. In the 16th century, the Brunei empire's influence extended as far as Kapuas River delta in West Kalimantan. The Malay Sultanate of Sambas in West Kalimantan and Sultanate of Sulu in Southern Philippines in particular developed dynastic relations with the royal house of Brunei. Other Malay sultans of Pontianak, Samarinda as far as Banjarmasin, treated the Sultan of Brunei as their leader. The true nature of Brunei's relations to other Malay Sultanates of coastal Borneo and Sulu archipelago is still a subject of study, as to whether it was a vassalised state, an alliance, or just a ceremonial relationship. Other regional polities also exercised their influence upon these sultanates.  The Sultanate of Banjar...

Question: Brunei's empire was in what part of West Kamimantan?
Kapuas River delta